runtime(vim): Update base-syntax, improve folding function matches

- Allow function command modifiers.
- Match function bodies starting with empty lines.

Command modifiers reported by @Konfekt.

fixes #15671
closes: #15674

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

runtime/syntax/testdir/dumps/vim_ex_def_05.dump

@@ -17,4 +17,4 @@
 @75
 @75
 |"+0#0000e05&| |c|o|m@1|e|n|t|s| +0#0000000&@64
-@57|9|0|,|0|-|1| @7|9|3|%| 
+@57|9|0|,|0|-|1| @7|8|2|%|