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>
diff --git a/runtime/syntax/testdir/input/vim_keymap.vim b/runtime/syntax/testdir/input/vim_keymap.vim
index 983c822..424d437 100644
--- a/runtime/syntax/testdir/input/vim_keymap.vim
+++ b/runtime/syntax/testdir/input/vim_keymap.vim
@@ -23,4 +23,5 @@
                                          
 " Vim-script comment characters                               
 # <char-0x00a3>                Line should not match as a Vim9-script comment
-\" “                   Line should not match as a legacy-script comment 
+\" “                           Line should not match as a legacy-script comment 
+: "                            Line should not match as a legacy-script comment