[FR] Lua funcs for chat buffer

How about this?

`Spring.GetUserWriting()` -> string input, number meaning
`Spring.GetUserRawWriting()` -> string raw_input, number meaning

Meaning would be one of (cast from char to int, lua can use `string.byte`):
* 'a' if ally chat ("a: foo")
* 's' if spectator chat ("s: foo")
* 'c' if command ("/foo")
* 'e' if everyone chat ("foo")
* nil if not writing

For example if the chat textbox currently has "a: blabla" then:
* `Spring.GetUserWriting()` returns "blabla", 'a'
* `Spring.GetUserRawWriting()` returns "a: blabla", 'a'

Rationale: often you would want to strip the magic prefix anyway, which is better to do in C++, which is why the version that strips away the prefix. The "meaning" should be cheap to get and is also usually good to know. Technically anything other than the raw input is redundant so I wouldn't bemoan their lack too much, but it's good if people aren't forced to reimplement the other two in Lua.

Start by locating the chat buffer implementation and the Lua binding entry points for the proposed Spring.GetUserWriting() and Spring.GetUserRawWriting() functions; the issue names no files or tests. Done means both functions distinguish ally, spectator, command, and everyone chat, return the specified stripped or raw text, return the meaning code, and return nil when the user is not writing.

Written by the indexing model from the issue text.