Add LKQL language highlight

[Heziode]

Context

Currently, it is a little bit a pain in the ass to develop complex rule using LKQL, without syntax highlight.

Solution

It could be nice to have a LKQL (LangKit Query Language) syntax highlight (and maybe auto-completion) into VSCode (into Ada extension or in a separate extension). it could have it's own LSP.

[raph-amiard]

Hello Heziode!

FWIW (and sorry), we have an highlighter for vim here: https://github.com/AdaCore/langkit-query-language/blob/master/utils/lkql.vim

It should be relatively easy to translate this to TMlanguage to accomodate VScode and others

OTOH for the completion/LSP part, we're still pretty far from that. One of the reasons is that for the moment LKQL is fully dynamic, and we want to introduce a type system in the near future, that will largely help in providing code intelligence features