runtime(vim): Update base-syntax, improve function definition highlighting (#14203)

runtime(vim): Update base-syntax, improve function definition highlighting (#14203)

Improve function definition highlighting.

- Match bang and function modifiers - abort etc.
- Only match valid scope modifiers.
- Match listing commands.
- Don't match ex commands in function names.
- Split function syntax groups into :func and :def subgroups.
- Match Vim9-script parameter and return types.
- Limit legacy-script and Vim9-script comments to :func and :def
  definitions, respectively.

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