runtime(vim): improve &keywordprg in ftplugin

runtime(vim): improve &keywordprg in ftplugin

- let keywordprg in vim filetype handle context-sensitive help calls by
  detecting the syntax group of the word under the cursor
- reformat whitespace
- add modeline

related: #16677
closes: #16680

Co-authored-by: Andrew Radev <andrey.radev@gmail.com>
Co-authored-by: "D. Ben Knoble" <ben.knoble+github@gmail.com>
Co-authored-by: Gary Johnson <garyjohn@spocom.com>
Co-authored-by: Tim Pope <code@tpope.net>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>
Co-authored-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Konfekt <Konfekt@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>