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_nested_fold.vim

@@ -0,0 +1,22 @@
+vim9script
+# Vim9 :def command (nested)
+# VIM_TEST_SETUP let g:vimsyn_folding = 'f'
+# VIM_TEST_SETUP setl fdc=2 fdm=syntax
+
+class Test
+    const name: string
+
+    def new()
+	def Name(): string
+	    function GiveName()
+		return "any"
+	    endfunction
+
+	    return GiveName()
+	enddef
+
+	this.name = Name()
+    enddef
+endclass
+
+echo Test.new()