cli: move get_new_config_file_path() to commands::config #16750

Job	Run time
build (ubuntu-latest)	5m 6s
Build jj-lib without Git support	45s
build (macos-13)	9m 22s
build (macos-14)	4m 24s
cargo-deny (advisories)	24s
build (windows-latest)	9m 48s
cargo-deny (bans licenses sources)	24s
Check protos	1m 22s
build (ubuntu-latest, --all-features)	6m 11s
Check that MkDocs can build the docs	9s
Check that MkDocs can build the docs with latest Python and uv	15s
Check formatting	20s
Clippy check	2m 38s
	41m 8s

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