Improve hover for function docs

[ManuelHentschel]

This PR tries to improve the quality of function documentation in hovers.

To test:
Hover e.g. over these expressions:

is.character()
is.na()
is.null()

Known issues:
Some special characters (e.g. the quotes in the documentation for is.na) get rendered incorrectly.
(Edit: fixed by specifying the encoding to be utf-8)

[renkun-ken]

Thanks for the work! The hover help looks much better.

I did some minor updates to the code to make it consistent with other usages in languageserver.

One issue I'm curious about is that it might be a bit heavy that we need to import rmarkdown which relies on pandoc to do the work. Do we already depend on it?

[renkun-ken]

I add a check rmarkdown::pandoc_available() to ensure that pandoc is available. If pandoc could be optional, then there might be no point to hard depend on rmarkdown anyway?

[randy3k]

As html_to_markdown could be quite heavy, shall we cache the markdown results (at least per session)?

[renkun-ken]

Agree. A simple dict to cache help results should be working.

[ManuelHentschel]

One issue I'm curious about is that it might be a bit heavy that we need to import rmarkdown which relies on pandoc to do the work. Do we already depend on it?

If pandoc could be optional, then there might be no point to hard depend on rmarkdown anyway?

I guess moving rmarkdwon to Suggests: and checking if it's installed before calling rmarkdown::pandoc_available() should work. If either of the checks fails we can still fall back to the old behaviour using repr::repr_text.

[renkun-ken]

LGTM

[renkun-ken]

@ManuelHentschel Just notice that gfm markdown format is not built-in by lower version of pandoc (e.g. pandoc 1.19.2.4 installed from official repo of Ubuntu 18.04).

Although the spec says it should follow gfm, I tried commonmark and markdown_strict and the output looks mostly the same.

Do we check the version of pandoc via rmarkdown::pandoc_version() and conditionally use gfm or commonmark? I'm not sure which version starts to build in gfm format.

[renkun-ken]

From the release notes of pandoc:

New input format gfm (GitHub-flavored CommonMark) (#3841). This uses bindings to GitHub’s fork of cmark. markdown_github has been deprecated in favor of gfm.

I guess we might need to use markdown_github for pandoc < 2.0 and gfm for pandoc >= 2.0.

[renkun-ken]

Fixed via 5f1d88e.