patch 8.2.2931: Vim9: line continuation comment uses legacy syntax

Problem:    Vim9: line continuation comment still uses legacy syntax in one
            place.
Solution:   Check for #\ instead of "\ earlier. (closes #8316)

src/testdir/test_vim9_script.vim

@@ -3081,6 +3081,9 @@ def Test_vim9_comment()
       \ start='foo'
       #\ comment
       \ end='bar'
+      syn region Text start='foo'
+      #\ comment
+      \ end='bar'
   END
   CheckScriptSuccess(lines)