runtime(vim): Update base-syntax, fix nested function folding (#14397)

Only match function folding start and end patterns at the start of a
line, excluding heredocs and :append/:change/:insert commands.

Fixes #14393

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

runtime/syntax/testdir/dumps/vim_ex_function_fold_03.dump

@@ -17,4 +17,4 @@
 | +0#0000000#ffffff0@74
 @75
 |"+0#0000e05&| |m|o|d|i|f|i|e|r|s| +0#0000000&@63
-@57|5@1|,|0|-|1| @7|2|8|%| 
+@57|5@1|,|0|-|1| @7|2@1|%|