syntax/sub_qc.vim: Highlight ASS dialogue line metadata
Set a separate colour for this metadata to differentiate it from the
dialogue content. Don't spell check this section as it's not part of the
dialogue.
This region contains the time code that we defined in
3c32a416c9a380a65835e17955c862a98a8d5cd7. Set `contained` on
`subQCTimeCode` because the time code only ever appears in this region.
We match 7 commas instead of 9 (as seen in the `NewEntry()` function in
ftplugin because the time code contains a comma and is preceded by a
comma.