feat: add scaffolding for merging migrations #1299

Job	Run time
Build & test (stable, ubuntu-latest)	37s
Build & test (stable, macos-latest)	56s
Build & test (stable, windows-latest)	40s
Build & test (nightly, ubuntu-latest)	55s
Build & test (nightly, macos-latest)	56s
Build & test (nightly, windows-latest)	41s
Build & test (1.84, ubuntu-latest)	41s
Build & test (1.84, macos-latest)	46s
Build & test (1.84, windows-latest)	40s
Clippy lint checks	0s
Test coverage checks	0s
Test with external dependencies	0s
Build with each feature combination	0s
Machete dependencies checks	0s
Minimal dependency versions build	0s
Miri test	0s
Code formatting checks	0s
	6m 52s

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