Signature Help

[3Rafal]

The goal of this PR is to upstream SignatureHelp functionality from ocaml-lsp to merlin. Similarly to #1286.

[3Rafal]

@voodoos, I've rebased this branch and it should be good for review

[voodoos]

Thanks Rafal !

[xvw]

Nice!
I made a first round of review (with maybe some nitpicking. Feel free to discard comments if you find it not relevant)

[voodoos]

Thanks Rafal, I think this covers well the first step which was to bring the code from ocaml-lsp to merlin-lib.

You should also document the new command in the protocol file: https://github.com/ocaml/merlin/blob/master/doc/dev/PROTOCOL.md

Aditionnaly, it looks like the LSP protocol also provides additional optional parameters to help the server provide a correct answer: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_signatureHelp

May be we should also accept these even if they are not used right now ?

[3Rafal]

@voodoos
even simpler case for this would be

It can make a third non-labelled parameter active.

  $ $MERLIN single signature-help -position 2:14 <<EOF
  > let f x y z = ignore (x, y, z)
  > let _ = f 0 1

result will be activeParameter equal one. As I understand it, the reason is that we search by arguments and when you just put the space after an argument, there are no new arguments in typed tree.