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_function_nested.vim

@@ -0,0 +1,38 @@
+" Vim :def and :function commands (nested)
+
+def FunA(): string
+    def DoFunA(): string
+	return "."
+    enddef
+
+    return DoFunA()
+enddef
+
+def FunB(): string
+    function DoFunB()
+	return ".."
+    endfunction
+
+    return DoFunB()
+enddef
+
+function FunC()
+    def DoFunC(): string
+	return "..."
+    enddef
+
+    return DoFunC()
+endfunction
+
+function FunD()
+    function DoFunD()
+	return "...."
+    endfunction
+
+    return DoFunD()
+endfunction
+
+echo FunA()
+echo FunB()
+echo FunC()
+echo FunD()