FR: Add new revset `builtin_immutable_heads()` #13248

Job	Run time
Build jj-lib without Git support	44s
cargo-deny (advisories)	21s
build (ubuntu-latest)	4m 35s
cargo-deny (bans licenses sources)	22s
build (macos-14)	3m 46s
Check protos	31s
build (windows-latest)	9m 0s
Clippy check	2m 35s
build (ubuntu-latest, --all-features)	5m 24s
Check that MkDocs can build the docs	23s
Check that MkDocs can build the docs with Poetry 1.8	21s
Check formatting	18s
	28m 20s

Provide feedback