Fix SGR mouse movement reports

[j4james]

According to the documentation, the final character of an SGR mouse
report is meant to be M for a button press and m for a button
release. However it isn't clear what the final character should be for
motion events, and we were using an m if there weren't any buttons
held down at the time, while all other terminals used an M, regardless
of the button state.

This PR updates our implementation to match what everyone else is doing,
since our interpretation of the spec was causing problems for some apps.

Validation Steps Performed

I've manually tested the new behavior in Vttest, and confirmed that our
mouse reports now match Xterm more closely, and I've tested with v0.42.0
of Zellij, which was previous glitching badly in Windows Terminal, but
now works correctly.

I've also updated our unit tests for the SGR mouse mode to reflect the
correct report format.

Closes #18712

[DHowett]

This feels completely correct. Thanks for maintaining our compatibilty and correctness :)

[DHowett]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[DHowett]

I've tried to kick the CLA robot... let's see if it works?

[DHowett]

IT WORKS