Shift + click gesture support for SelectionArea on desktop platforms

[Renzo-Olivares]

Shift + Click to move the selection end edge on desktop platforms. This is consistent with native Linux and Windows. On macOS the behavior moves the selection edge closest to the tapped position (will implement in a later PR).

Part of: #129583

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or this PR is [test-exempt].
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