Fix the Alternate Scroll Mode when DECCKM enabled

[j4james]

Summary of the Pull Request

If the Alternate Scroll Mode is enabled, the terminal generates up/down keystrokes when the mouse wheel is scrolled. However, the expected escape sequences for those keys are dependent on the state of the Cursor Keys Mode ( DECCKM), but we haven't taken that into account. This PR updates the alternate scroll implementation to make sure the appropriate sequences are sent for both DECCKM modes.

References

#3321

PR Checklist

• Closes #xxx
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

I've simply added a condition in the TerminalInput::_SendAlternateScroll method to send a different pair of sequences dependent on the state of _cursorApplicationMode flag.

Validation Steps Performed

Manually tested in VIM (although that required me enabling the Alternate Scroll Mode myself first). Also added a new unit test in MouseInputTest to confirm the correct sequences were generated for both DECCKM modes.

[DHowett-MSFT]

Thanks so much for this.

[zadjii-msft]

Thanks!

[ghost]

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