Implements #52 - SciTE text editor load support

Does NOT implement save.
Only supports Tombo CHI files.
Attempts to save *.chi files will be blocked to prevent replacement with
plain text, does NOT do so for other files (but also doesn't attempt to
decrypt them either).

Requires operating system environment variable PT_PASSWORD is set before
loading scite.
Requires ptcipher in path or set os env PTCIPHER_EXE to full path

Can be copy/pasted into Scite Lua start up script or if using
ParsKorata mini/simple ExtMan can add:

    mgr:load_files{'puren_tonbo.lua'}

to Lua startup script.

pt_scite.lua

@@ -0,0 +1,109 @@
+-- SciTE Lua Puren Tonbo script for reading encrypted files
+-- https://www.scintilla.org/SciTE.html
+-- https://github.com/clach04/puren_tonbo
+-- Can be used standalone or with ParsKorata (http://lua-users.org/wiki/ParsKorata) mini/simple ExtMan compatible script
+-- Untested with ExtMan - http://lua-users.org/wiki/SciteExtMan
+
+-- Only supports Tombo format *.chi files
+-- Usage, ensure:
+--  1) operating system environment variable PT_PASSWORD is set before loading scite
+--  2) that ptcipher is in path or set os env PTCIPHER_EXE to path
+
+-- http://lua-users.org/wiki/StringRecipes
+local function starts_with(str, start)
+   return str:sub(1, #start) == start
+end
+
+local function ends_with(str, ending)
+   return ending == "" or str:sub(-#ending) == ending
+end
+
+-- TODO encrypted filename determination PTCIPHER_EXE, '--list-formats', '--no-prompt'
+-- local prog = PTCIPHER_EXE .. ' --list-formats --no-prompt'
+-- local f = io.popen(prog, 'rb')  -- read
+-- program_output = f:read('*a')  -- read entire file
+-- print(program_output)
+-- local popen_success = f:close()  -- this is nil or boolean, not integer  Lua 5.2 feature?
+-- print('popen_success: ' .. tostring(popen_success))
+
+-- NOTE SaveEncryptedFile() does not (yet) support saving encrypted files, instead it prevents accidental saving
+local function SaveEncryptedFile(filename)
+  --print('OnBeforeSave')  -- to output pane
+  --print(filename)  -- to output pane
+  filename_lower = string.lower(filename)
+  if ends_with(filename_lower, '.chi') then -- Tombo file, TODO more formats
+    --print('block CHI save')  -- to output pane
+    --print(filename)  -- to output pane
+    print('blocked CHI save ' .. filename)  -- to output pane
+    --editor:EmptyUndoBuffer()
+    --editor.UndoCollection=1
+
+    --editor:SetSavePoint() -- indicate to editor that save happened - whether it really did or not ;-)
+    return true -- indicate to editor NOT to save
+  end
+end
+
+-- parskorata_extman.lua does NOT have scite_OnBeforeSave()
+if scite_OnBeforeSave~=nil then
+  scite_OnBeforeSave(SaveEncryptedFile)
+else
+    -- avoid: attempt to call a nil value (global 'scite_OnBeforeSave')
+    function OnBeforeSave(filename)
+      return SaveEncryptedFile(filename)
+    end  -- OnBeforeSave
+end
+
+local function LoadEncryptedFile(filename)
+  --print('OnOpen')  -- to output pane
+  --print(filename)  -- to output pane
+  filename_lower = string.lower(filename)
+  if ends_with(filename_lower, '.chi') then -- Tombo file
+    print('DEBUG block CHI load ' .. filename)  -- to output pane
+    --editor:EmptyUndoBuffer()
+    --editor.UndoCollection=1
+    --editor:SetSavePoint() -- indicate to editor that save happened - whether it really did or not ;-)
+    --return false  - reverse value compared with before save, false tells editor to NOT load and to NOT open a new pane
+    -- either return false and open new tab and populate OR replace content in tab
+
+    local PTCIPHER_EXE = os.getenv('PTCIPHER_EXE') or 'ptcipher'
+
+    -- TODO SciTE 4.4.4 on Windows adds create.hidden.console option to stop console window flashing when Lua script calls os.execute or io.popen.https://groups.google.com/g/scite-interest/c/QOhizNSEejU/m/qXslloxnCgAJ\
+    -- getenv works, there is no setenv unless using posix stdlib extension http://luaposix.github.io/luaposix/modules/posix.stdlib.html#setenv
+    -- os.environ['PT_PASSWORD'] = 'bad'
+    -- NOTE expects PT_PASSWORD to be set, there is no way to set this from lua and do not want to use command line arg to pass password
+    local prog = PTCIPHER_EXE .. ' --decrypt "' .. filename .. '" --output=- --no-prompt'
+    local f = io.popen(prog, 'rb')  -- read
+    program_output = f:read('*a')  -- read entire file
+    -- TODO FIXME if program_output is empty treat as an error
+    --print(program_output)
+    local popen_success = f:close()  -- this is nil or boolean, not integer  Lua 5.2 feature?
+    editor:SetText(program_output)  -- this may be real content (decrypted text) or error text (e.g. bad password), check popen_success to determine
+    if popen_success == true then
+        -- if no error
+        editor:SetSavePoint()  -- indicate to editor that save happened and file is unchanged - whether it really did or not ;-)
+    else
+        -- error handling
+        print('popen_success: ' .. tostring(popen_success))
+        -- either nil or false - so far only seen nil for both; failure to launch (missing PTCIPHER_EXE) and also exe launched and returned errors (like bad password)
+        -- empty output seen for missing exe and also missing file - TODO consider adding extra output to ptcipher for missing file case
+        print('failed to load using '.. prog)
+        print('failed to load using '.. PTCIPHER_EXE)
+        --editor:AddText('failed to load using '.. PTCIPHER_EXE)
+        editor:AppendText('\nfailed to load using '.. prog)
+        editor:AppendText('\nSuggestions, check:\n')
+        editor:AppendText('  1) file exists\n')
+        editor:AppendText('  2) environment variable PTCIPHER_EXE - ptcipher script/executable\n')
+        editor:AppendText('  3) environment variable PT_PASSWORD - password\n')
+    end  -- error handling
+  end  -- encrypted file
+end  -- LoadEncryptedFile
+
+-- parskorata_extman.lua does NOT have scite_OnBeforeSave()
+if scite_OnOpen ~= nil then
+  scite_OnOpen(LoadEncryptedFile)
+else
+    -- avoid: attempt to call a nil value (global 'scite_OnOpen')
+    function OnOpen(filename)
+      return LoadEncryptedFile(filename)
+    end  -- OnOpen
+end