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-vim9.vim

@@ -0,0 +1,41 @@
+vim9script
+
+# Vim comments
+
+# Vim9-script comment
+
+ # string only recognised with leading char
+ "useless string"
+
+function! Foo()
+  " Legacy-script comment
+  # 42 " comment
+endfunction
+
+def! Bar()
+  # Vim9-script comment
+  "useless string" # comment
+enddef
+
+command -count 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