ui: redirect prompt messages to stderr #14772

Job	Run time
build (ubuntu-latest)	4m 34s
build (macos-13)	10m 29s
build (macos-14)	4m 6s
build (windows-latest)	9m 28s
build (ubuntu-latest, --all-features)	5m 31s
Build jj-lib without Git support	45s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	25s
Check protos	25s
Clippy check	2m 29s
Check that MkDocs can build the docs	19s
Check that MkDocs can build the docs with Poetry 1.8	20s
Check formatting	16s
	39m 31s

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