This PR adds support for the `DECRQSS` (Request Selection or Setting)
escape sequence, which is a standard VT query for reporting the state of
various control functions. This initial implementation only supports
queries for the `DECSTBM` margins, and the `SGR` graphic rendition
attributes.
This can be useful for certain forms of capability detection (#1040). As
one example in particular, it can serve as an alternative to the
`COLORTERM` environment variable for detecting truecolor support
(#11057).
Of the settings that can be queried by `DECRQSS`, the only other one
that we could be supporting at the moment is `DECSCUSR` (Cursor Style).
However, that would require passing the query through to the conpty
client, which is a lot more complicated, so I thought it best to leave
for a future PR.
For now this gets the basic framework in place, so we are at least
responding to queries, and even just supporting the `SGR` attributes
query is useful in itself.
Validation
----------
I've added a unit test verifying the reports for the `DECSTBM` and `SGR`
settings with a range of different parameters. I've also tested the
`DECSTBM` and `SGR` reports manually in _Vttest_, under menu 11.2.5.3.6
(Status-String Reports).
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