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/input/vim_ex_def.vim

@@ -107,3 +107,15 @@ def Foo()
   "useless string"
 enddef
 
+
+" leading command separator
+
+echo "Foo" | def Foo()
+enddef
+
+
+" command modifiers
+
+silent! def Foo()
+enddef
+