runtime(vim): Update base-syntax, add legacy header folding

Allow for syntax-based folding of Vim9 script legacy header regions.

This is enabled with the "H" flag of the g:vimsyn_folding config variable.

closes: #14530

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

runtime/syntax/testdir/input/vim9_legacy_header_fold.vim

@@ -0,0 +1,17 @@
+" VIM_TEST_SETUP let g:vimsyn_folding = "H" | set fdm=syntax
+" comment
+
+if !has('vim9script')
+  # 42 " comment
+  source foo.vim
+  finish
+endif
+
+" comment
+
+vim9script noclear
+
+# comment
+
+ # string only recognised with leading char
+ "useless string"