runtime(vim): Update base-syntax, improve function definition matching

- Fix highlighting of function names including /fu\%[nction]/ (E.g.,
  s:func(), foo.fu(), fu.func())
- Match :delfunction argument.

Reported by Aliaksei Budavei.

closes: #17428

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

runtime/syntax/testdir/dumps/vim_ex_function_02.dump

@@ -17,4 +17,4 @@
 @2|r+0#af5f00255&|e|t|u|r|n| +0#0000000&|4+0#e000002&|2| +0#0000000&@63
 |e+0#af5f00255&|n|d|f|u|n|c|t|i|o|n| +0#0000000&@63
 @75
-@57|3|7|,|0|-|1| @7|1|4|%| 
+@57|3|7|,|0|-|1| @7|1|2|%|