runtime(vim): Fix for :VimKeywordPrg when syntax does not match

runtime(vim): Fix for :VimKeywordPrg when syntax does not match

When using vim9-syntax plugin, :VimKeywordPrg does not lookup functions
correctly, as it relies solely on syntax names to find the help topic.

The syntax keyword used for builtin function is vi9FuncNameBuiltin in
vim9-syntax plugin, not vimFuncName expected by :VimKeywordPrg, so the
fallback rules apply, and there is no fallback rule for function calls.

Fix by just checking if the first char after topic is '(', and if so
assume help topic is a function.

closes: #19320

Signed-off-by: Mark Woods <mwoods.online.ie@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>