ui: redirect prompt messages to stderr #14568

Job	Run time
Build jj-lib without Git support	45s
build (ubuntu-latest)	4m 28s
build (macos-13)	6m 19s
cargo-deny (advisories)	24s
build (macos-14)	3m 41s
Check protos	24s
Clippy check	2m 31s
cargo-deny (bans licenses sources)	23s
build (windows-latest)	8m 43s
build (ubuntu-latest, --all-features)	5m 35s
Check that MkDocs can build the docs	29s
Check that MkDocs can build the docs with Poetry 1.8	26s
Check formatting	17s
	34m 25s

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