runtime(vim): Update help syntax file, improve highlighting of included Vim examples

runtime(vim): Update help syntax file, improve highlighting of included Vim examples

- Take over as file maintainer.
- Improve highlighting of legacy script examples by using :syn-iskeyword
  with the default 'iskeyword' value. Vim9 script examples are not
  supported yet.
- Match admonition labels in more contexts.
- Match URLs in more contexts.

fixes #17721
closes: #17731

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>