implement language server stub

[endorama]

Add a stub implementation of ES|QL language server for VSCode.

This is an experimental implementation and is not fully working (details of what works and what not are included in the folder README).

There is a major difference in the implementation compared to the Kibana plugin achieving the same functionality. They both leverage ANTLR4 grammar but Kibana is using antlr4ts while this implementation uses antlr4ng to leverage https://github.com/mike-lischke/antlr4-c3.

[dej611]

Ideally we should all use the same antlr4 tool. There's a task in Kibana to migrate the antlr4 library to use the same library as Elasticsearch with the typescript target to stay always in sync: elastic/kibana#172199

[smith]

I can give you write access if you want to merge this and work off of main. Whatever works for you. Thanks for doing this!

[endorama]

@dej611 thanks for chiming in. I agree, it does not make sense to have different implementations. What I would also like to leverage, as that is the bulk of the implementation, is the auto-completion logic that is currently in the Kibana plugin. I think switching to core anltr4 does not present any issue but would remove the current (very small so not much of a concern) auto-completion functionalities: I choose antlr4ng to get a working prototype faster thanks to the antlr4-c3 auto-completion engine that goes on pair with it, so the current auto-completion relies on it.

@smith I don't think it make sense to continue moving forward this branch with antlr4ng if the Kibana plan is to use plain antrl4, but I also don't think it make sense to duplicate the logic. I would consider completing elastic/kibana#173612 a prerequisite to move forward with something that has enough features to be useful and does not poses an additional big maintenance burden on the language server implementation itself.