ocamllsp: add more filetypes

These are ocamlinterface, ocamllex and menhir.

This is in preparation when these filetypes will be introduced in order
to keep lsp working on them. The change is backwards compatible.

For more information about the rationale of this change please read
ocaml/vim-ocaml#61

lua/lspconfig/ocamllsp.lua

@@ -2,7 +2,10 @@ local configs = require 'lspconfig/configs'
 local util = require 'lspconfig/util'
 
 local language_id_of = {
+  menhir = 'ocaml.menhir';
   ocaml = 'ocaml';
+  ocamlinterface = 'ocaml.interface';
+  ocamllex = 'ocaml.ocamllex';
   reason = 'reason';
 }