Improve handling of comments and doc comments

[wchargin]

Doc comments are now properly formatted with gq and J. For example,
on this input text:

/// one
/// two

pressing J on the top line used to result in /// one / two, and now
results in /// one two. Conversely, on this input text:

/// this long line exceeds the text width

pressing gqgq with a textwidth of 20 now yields:

/// this long line
/// exceeds the text
/// width

instead of:

/// this long line
//exceeds the text
//width

Furthermore, adding a new line to an existing comment, by pressing o
in normal mode or Enter in insert mode, now automatically inserts the
appropriate comment leader (// or ///).

wchargin-branch: comment-doc-and-leader

[chanhosuh]

@wchargin how about also accommodating the /**.. **/ natspec style? ;)

[wchargin]

@wchargin how about also accommodating the /**.. **/ natspec style? ;)

Go for it! I personally don't use that natspec style and am not super
familiar with this corner of Vim config, so I'm not going to implement
this myself.

[mhluongo]

Hey @wchargin, I merged this PR in our maintained fork!