Add hover support for warning attributes

[ulugbekna]

A user hovering over [@warning "-27"] could see that this attribute is disabling warning 27 and description of what that warning is

[xvw]

To achieve this, you would need to refer to the function integrated via this issue: ocaml/merlin#1775 for linking between warning id and warning description.

The hover functionality is a standard LSP request, specified here. On the ocaml-lsp-server side, the functionality is implemented in the Hover_req module. The entry point is the hover_at_cursor function. which iterates over each element of the AST. The aim of the issue is to add a case for the attributes identified by @warning and to use their index to return a textual formatting, which will be provided via the function added via ocaml/merlin#1775.

Feel free to ask more clarification!