Add support for DECARM (Auto Repeat Mode)

[j4james]

trafficstars

Summary of the Pull Request

This PR adds support for the DECARM (Auto Repeat Mode) sequence, which controls whether a keypress automatically repeats if you keep it held down for long enough.

References

Note that this won't fully work in Windows Terminal until issue #8440 is resolved.

PR Checklist

• [x] Closes #13919
• [x] CLA signed.
• [x] Tests added/passed
• [ ] Documentation updated.
• [ ] Schema updated.
• [x] I've discussed this with core contributors already. Issue number where discussion took place: #13919

Detailed Description of the Pull Request / Additional comments

Every time we receive a KeyDown event, we record the virtual key code to track that as the last key pressed. If we receive a KeyUp event the matches that last key code, we reset that field. Then if the Auto Repeat Mode is reset, and we receive a KeyDown event that matches the last key code, we simply ignore it.

Validation Steps Performed

I've manually tested the DECARM functionality in Vttest and confirmed that it's working as expected. Although note that in Windows Terminal this only applies to non-alphanumeric keys for now (e.g. Tab, BackSpace, arrow keys, etc.)

I've also added a basic unit test that verifies that repeated key presses are suppressed when the DECARM mode is disabled.