Add support for querying the DECAC settings

[j4james]

This PR adds support for querying the color indices set by the DECAC
control, using the existing DECRQSS implementation.

References and Relevant Issues

The initial DECRQSS support was added in PR #11152.
The DECAC functionality was added in PR #13058, but at the time we
didn't know how to format the associated DECRQSS query.

Detailed Description of the Pull Request / Additional comments

For most DECRQSS queries, the setting being requested is identified by
the final characters of its escape sequence. However, for the DECAC
settings, you also need to include a parameter value, to indicate which
color item you're querying.

This meant we needed to extend the DECRQSS parser, so I also took this
opportunity to ensure we correctly parsed any parameter prefix chars. We
don't yet support any setting requiring a prefix, but this makes sure we
don't respond incorrectly if an app does query such a setting.

Validation Steps Performed

Thanks to @al20878, we've been able to test how these queries are parsed
on a real VT525 terminal, and I've manually verified our implementation
matches that behavior.

I've also extended the existing DECRQSS unit test to confirm that we
are responding to the DECAC queries as expected.

PR Checklist

• Closes Add support for querying DECAC with DECRQSS #13091
• Tests added/passed
• Documentation updated
• Schema updated (if necessary)

[j4james]

I think the CI test failure is just the flaky feature test timeout.

All good now.

[DHowett]

I love it. Thank you! :)