console: configure continuation

Continuation marker can be set up with `\set continuation` command.
Works on both server and client side in any language.

Closes tarantool#4317
Requires tarantool#7357

@TarantoolBot document
Title: introduce line carrying slash

Now we can use multiline commands with lines ending by configuring
continuation symbol. Works only when there is no set delimiter.
There is also an alias `\set c on|off`, that does the same.
Consider the example where the marker is set, used and removed:
```
tarantool> \set continuation on
---
- true
...

tarantool> a = 10\
         > + 12
---
...

tarantool> \set c off
---
- true
...

tarantool> a = 10\
---
- error: '[string "a = 10\"]:1: unexpected symbol near ''\'''
...

tarantool>

```

src/box/lua/console.lua

@@ -56,6 +56,9 @@ local continuation_symbol = '\\'
 
 local default_local_eos = ''
 
+-- Each but the last string of multiline command should end with this string.
+local continuation_symbol = '\\'
+
 output_handlers["yaml"] = function(status, _opts, ...)
     local err, ok, res
      -- Using pcall, because serializer can raise an exception
@@ -593,7 +596,7 @@ local function wrap_text_socket(connection, url, print_f)
         eos = current_eos(),
         fmt = current_output()["fmt"],
         local_eos = default_local_eos,
-        continuation_on = false,
+        continuation_on = false
     }, text_connection_mt)
     --
     -- Prepare the connection: setup EOS symbol