Add support for querying the DECSCUSR setting

[j4james]

Summary of the Pull Request

This PR adds support for querying the cursor style - technically the
state of the DECSCUSR setting - using a DECRQSS escape sequence.

References and Relevant Issues

The initial DECRQSS support was added in PR #11152, but it wasn't
practical to report the cursor style until conpty passthrough was added
in PR #17510.

Detailed Description of the Pull Request / Additional comments

If the user has chosen a cursor style that isn't one of the shapes
supported by the DECSCUSR control, we report those as 0 (i.e. the
default style). That way, if an application later tries to restore the
cursor using the returned value, it should still be reset to its
original state.

I also took the opportunity in this PR to do some refactoring of the
other DECRQSS reports, since several of them were using unnecessary
appending that could be simplified to a single fmt::format call, or
even just static strings in some cases.

Validation Steps Performed

I've checked the reports are working as expected in Vttest, and also
added some unit tests.

PR Checklist

• Tests added/passed

[DHowett]

Thanks!