runtime(vim): Improve Vim9 and legacy-script comment highlighting (#13104)

This is a first-pass attempt to limit matching of Vim9 and legacy-script
comments to the appropriate syntactic contexts.

Vim9-script comments are highlighted at top level in a Vim9-script file,
in all :def functions, and in all :autocmd and :commmand command blocks.
Legacy-script comments are highlighted at top level in a legacy script
file, in all :func functions and in the Vim9-script preamble before the
:vim9script marker command.

Fixes #13047, #11307 and #9587.

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

runtime/syntax/testdir/input/vim_ex_commands.vim

@@ -558,7 +558,7 @@
 :verbose
 :vertical
 :vim9cmd
-:vim9script
+" :vim9script
 :vimgrep
 :vimgrepadd
 :visual
@@ -1152,7 +1152,7 @@ call Foo() | version
 call Foo() | verbose
 call Foo() | vertical
 call Foo() | vim9cmd
-call Foo() | vim9script
+" call Foo() | vim9script
 call Foo() | vimgrep
 call Foo() | vimgrepadd
 call Foo() | visual