tools: add `diffedit3` as a dotslash tool #13396

Job	Run time
build (ubuntu-latest)	4m 28s
build (macos-13)	4m 53s
build (macos-14)	4m 9s
build (windows-latest)	8m 49s
build (ubuntu-latest, --all-features)	5m 34s
Build jj-lib without Git support	45s
Check protos	26s
Check that MkDocs can build the docs	22s
Check that MkDocs can build the docs with Poetry 1.8	19s
Check formatting	16s
cargo-deny (advisories)	22s
cargo-deny (bans licenses sources)	44s
Clippy check	2m 38s
	33m 45s

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