Add Clippit as an optional comparison engine #18

Job	Run time
build-core	16s
build-engine-wheels (windows-latest, win-x64 win-arm64)	2m 51s
build-engine-wheels (macos-latest, osx-x64 osx-arm64)	1m 35s
build-engine-wheels (ubuntu-latest, linux-x64 linux-arm64)	1m 52s
test (ubuntu-latest, 3.12)	1m 34s
test (windows-latest, 3.12)	2m 30s
test (windows-latest, 3.9)	3m 7s
test (ubuntu-latest, 3.9)	1m 40s
test (windows-latest, 3.11)	2m 16s
test (macos-latest, 3.11)	1m 2s
test (macos-latest, 3.12)	1m 16s
test (macos-latest, 3.9)	1m 24s
test (ubuntu-latest, 3.11)	1m 24s
	22m 47s

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