Add "Copy hunk to opposite side" actions to the Compare Marked Files diff viewer.

[gvozdetsky]

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Similar to IntelliJ IDEA's "Accept Left/Right" or VS Code's "Copy to Left/Right".

This would allow selectively copying a diff hunk from one compared file to the other without manual copy/paste.

This should be a generic capability of Zed's editable diff viewer rather than specific to Compare Marked Files. The same actions (Copy hunk to left/right) would also be useful in editor: diff clipboard with selection and any future two-way diff views.