Add support to enable lsp completion inside the OCaml blocks for OCamllex files

[smorimoto]

[smorimoto]

@rgrinberg @voodoos Where should this be addressed? Should we support this on the Merlin side?

[smorimoto]

Well, I'm sure this shouldn't be addressed on the extension side.

[mlantas]

Where should this be addressed? Should we support this on the Merlin side?

I think both the language server and Merlin would have to add support. If I'm not mistaken, LSP support would have to be registered for the entire mll filetype because embedded blocks are not supported by the protocol. Merlin would have to recognize regex variables defined in the lexer rules: https://caml.inria.fr/pub/docs/manual-ocaml/lexyacc.html#ss:ocamllex-variables

[smorimoto]

Yes, and it would be great if something like ocamlformat supports that, as well. I don't know if it should be done through LSP or by ocamlformat itself, but I think we need to take that into account.