Add a new node for Doc comments

[aspeddro]

ReScript v10.1 introduced docstrings for values, types, and modules.

• /**Docstring*/: before value and types
• /***Toplevel docstrins*/: top level modules

The two comments above are parsed as comment.

We can create two nodes (doc_comment and mod_comment) for both types of comments. It would be useful to do markdown injection.