build

config: replace LayeredConfigs with StackedConfig #130

Job	Run time
Build jj-lib without Git support	46s
cargo-deny (advisories)	22s
build (ubuntu-latest)	5m 2s
cargo-deny (bans licenses sources)	24s
Check protos	26s
build (macos-13)	7m 32s
build (macos-14)	4m 11s
Clippy check	2m 42s
build (windows-latest)	11m 4s
Check that MkDocs can build the docs	9s
build (ubuntu-latest, --all-features)	6m 0s
Check that MkDocs can build the docs with latest Python and uv	14s
Check formatting	16s
	39m 8s

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