config: migrate underlying data structure to toml_edit #16984

Job	Run time
Build jj-lib without Git support	47s
build (ubuntu-latest)	5m 23s
cargo-deny (advisories)	25s
build (macos-13)	7m 54s
cargo-deny (bans licenses sources)	28s
Clippy check	2m 35s
build (macos-14)	4m 13s
Check protos	26s
build (windows-latest)	10m 53s
Check that MkDocs can build the docs	14s
Check that MkDocs can build the docs with latest Python and uv	21s
build (ubuntu-latest, --all-features)	7m 49s
Check formatting	18s
	41m 46s

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