Add GUI option for new Syntax Documentation command

[PizieDust]

Related to ocaml/ocaml-lsp#1218

A new Syntax Document command is being implemented in Merlin which will display some information about the OCaml syntax under the cursor, such as where to find information about it on the official OCaml documentation website.
This feature will be a good wingman for complete beginners in OCaml and will save them from having to Google around before finding some helpful info or searching obscure symbols.

This feature will be made optional so that users can activate or deactivate it at will.
To make a truly great experience, it will be great to add it to this vs-code extension as a configurable GUI option so users can easily activate/deactivate it, the same way we do with codelens and extendedHover

cc @pitag-ha @voodoos