patch 8.1.0071: terminal debugger only works with the terminal feature

Problem:    Terminal debugger only works with the terminal feature.
Solution:   Make it also work with a prompt buffer.  Makes it possible to use
            on MS-Windows. Various other improvements. (closes #3012)

runtime/doc/terminal.txt

@@ -38,6 +38,7 @@ If the result is "1" you have it.
      Stepping through code		|termdebug-stepping|
      Inspecting variables		|termdebug-variables|
      Other commands			|termdebug-commands|
      Prompt mode			|termdebug-prompt|
      Communication			|termdebug-communication|
      Customizing			|termdebug-customizing|

@@ -620,6 +621,10 @@ The Terminal debugging plugin can be used to debug a program with gdb and view
the source code in a Vim window.  Since this is completely contained inside
Vim this also works remotely over an ssh connection.

When the |+terminal| feature is missing, the plugin will use the "prompt"
buffer type, if possible.  The running program will then use a newly opened
terminal window.  See |termdebug-prompt| below for details.


Starting ~
							*termdebug-starting*
@@ -799,6 +804,23 @@ Other commands ~
	     isn't one


Prompt mode ~
						*termdebug-prompt*
When the |+terminal| feature is not supported and on MS-Windows, gdb will run
in a buffer with 'buftype' set to "prompt".  This works slightly differently:
- The gdb window will be in Insert mode while typing commands.  Go to Normal
  mode with <Esc>, then you can move around in the buffer, copy/paste, etc.
  Go back to editing the gdb command with any command that starts Insert mode,
  such as `a` or `i`.
- The program being debugged will run in a separate window.  On MS-Windows
  this is a new console window.  On Unix, if the |+terminal| feature is
  available a Terminal window will be opened to run the debugged program in.

						*termdebug_use_prompt*
Prompt mode can be used even when the |+terminal| feature is present with: >
	let g:termdebug_use_prompt = 1


Communication ~
						*termdebug-communication*
There is another, hidden, buffer, which is used for Vim to communicate with
@@ -836,6 +858,14 @@ When 'background' is "dark":
  hi debugBreakpoint term=reverse ctermbg=red guibg=red


Shorcuts						*termdebug_shortcuts*

You can define your own shortcuts (mappings) to control gdb, that can work in
any window, using the TermDebugSendCommand() function.  Example: >
	map ,w :call TermDebugSendCommand('where')<CR>
The argument is the gdb command.


Popup menu						*termdebug_popup*

By default the Termdebug plugin sets 'mousemodel' to "popup_setpos" and adds