feat: add `migrate` subcmd. #305

Job	Run time
block-autosquash-commits	0s
cargo-deny	19s
Check (ubuntu-latest, stable)	30s
Rustfmt (stable)	13s
Check (ubuntu-latest, beta)	33s
Check (windows-latest, stable)	2m 18s
Check (windows-latest, beta)	1m 26s
Check (macOS-latest, stable)	36s
Check (macOS-latest, beta)	39s
Documentation (stable)	32s
Clippy (stable)	33s
Test Suite (ubuntu-latest, stable)	54s
Clippy (beta)	33s
Documentation (beta)	31s
Test Suite (ubuntu-latest, beta)	52s
Test Suite (windows-latest, stable)	3m 4s
Test Suite (windows-latest, beta)	2m 28s
Test Suite (macOS-latest, stable)	49s
Test Suite (macOS-latest, beta)	55s
	17m 45s

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