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_comment.vim

@@ -0,0 +1,45 @@
+" Vim comments
+
+" Legacy-script comment
+
+# 42 " comment
+
+function! Foo()
+  " Legacy-script comment
+  # 42 " comment
+endfunction
+
+def! Bar()
+  # Vim9-script comment
+  "useless string" # comment
+enddef
+
+command -nargs=1 FooCommand {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+autocmd BufNewFile * {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+
+" Issue: #13047
+
+if !exists(":DiffOrig")
+  command DiffOrig vert new | set bt=nofile | r ++edit # | 0d_ | diffthis
+		  \ | wincmd p | diffthis
+endif
+
+
+" Issue: #11307 and #11560
+
+" This is what we call " blah
+
+
+" Issue # #9587
+
+def CommentTitle()
+  # Title: ...
+enddef