runtime(vim): Update base-syntax, improve :echo and :execute highlighting (#14199)

Improve :echo and :execute highlighting.

- Add better line-continuation support for both commands.
- Improve the :execute command's expression argument matching.
- Remove the fix for issue #9987 as this is now handled by correctly
  matching the parens in :echo (...) as operator parens.


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

runtime/syntax/testdir/dumps/vim_ex_execute_02.dump

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