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/input/vim_ex_def_fold.vim

@@ -108,3 +108,23 @@ def Foo()
   "useless string"
 enddef
 
+
+" fold-region ending
+
+def Foo()
+  # enddef
+enddef
+
+def Foo()
+  echo "enddef"
+enddef
+
+def Foo()
+  let x =<< END
+    endfunction
+  END
+enddef
+
+:def Foo()
+:enddef
+