Convert roxygen comments to documentation

[renkun-ken]

Close #304

This PR imports roxygen2 and implements convert_comment_to_documentation where roxygen2::parse_text() is tried to parse comments as roxygen tags to provide structured documentation.

The information presented in hover contains all parsed roxygen tags presented in markdown format while only description is presented in completion resolve of symbol and function arguments and function signature, which is consistent with currently presented information provided by these providers.

Following is a screencast:

[renkun-ken]

There are several known limitations currently:

• The Rd tags (e.g. \code{}, \emph{}, \url{}) are not handled. roxygen2md could convert the tags but I'd rather leave them unhandled at the moment to keep the implementation clean.
• R6 roxygen tags could be non-continuous lines of roxygen comments. I don't handle such case at the moment but only take the comment lines above the symbol definition, like other symbols.

I believe this PR is ready to merge. We could always improve the implementation if requested.