Add syntax based folding for block comments

[dkearns]

This also enables folding of multiple consecutive line comments as a
single block.

[benknoble]

@dkearns I've attempted to merge this to https://github.com/benknoble/vim-racket (see results in https://github.com/benknoble/vim-racket/tree/merge-69). Can you let me know if that merge looks correct to you? I'll test drive it a bit, too.

In particular, look at 32d4343 (Merge syntax based folding for block comments, 2025-09-10).

[benknoble]

From a quick drive, it looks like it works to me for folding. I didn't try the "no_fold" override.

[benknoble]

I did have to push one fix to the merge-69 temporary branch for multiline comments:

range-diff: re-include form comments in racketTop

1:  b2a0b11 = 1:  b2a0b11 Add syntax based folding for block comments
2:  32d4343 ! 2:  f251a43 Merge syntax based folding for block comments
    @@ Commit message
               - 9f9a3ae (Use unabbreviated 'syntax' command name, 2022-08-11)
     
         PR: https://github.com/wlangstroth/vim-racket/pull/69
    +    Best-viewed-with: --diff-merges=1
     
      ## syntax/racket.vim ##
     @@ syntax/racket.vim: syntax match racketUnquote ",@"
    @@ syntax/racket.vim: syntax match racketUnquote ",@"
      syntax match racketComment /;.*$/ contains=racketTodo,racketNote,@Spell
     -syntax region racketMultilineComment start=/#|/ end=/|#/ contains=racketMultilineComment,racketTodo,racketNote,@Spell
      syntax match racketFormComment "#;" nextgroup=@racketTop
    - 
    ++syntax cluster racketTop add=racketFormComment
    ++
     +if exists("racket_no_comment_fold")
     +  syntax region racketBlockComment start=/#|/ end=/|#/ contains=racketBlockComment,racketTodo,racketNote,@Spell
     +else
     +  syntax region racketBlockComment start=/#|/ end=/|#/ contains=racketBlockComment,racketTodo,racketNote,@Spell fold
     +  syntax region racketMultilineComment start="^\s*;" end="^\%(\s*;\)\@!" contains=racketComment transparent keepend fold
     +endif
    -+
    + 
      syntax match racketTodo /\C\<\(FIXME\|TODO\|XXX\)\ze:\?\>/ contained
      syntax match racketNote /\CNOTE\ze:\?/ contained
      

[benknoble]

I've gone ahead and pushed my merge of this to benknoble/vim-racket's master branch.