Unify Prose and Textbox with an underlying `TextRegion` #2222

Job	Run time
cargo check (msrv) (wasm32)	16s
cargo check (aarch64-android)	24s
cargo clippy (windows-latest)	3m 14s
cargo check (msrv) (windows-latest)	1m 12s
cargo clippy (macos-latest)	1m 29s
cargo clippy (wasm32)	1m 17s
cargo check (msrv) (macos-latest)	51s
cargo clippy (ubuntu-latest)	2m 20s
cargo doc	44s
cargo check (msrv) (ubuntu-latest)	50s
formatting	37s
Prime LFS Cache	11s
cargo test (wasm32)	25s
typos	10s
cargo test (windows-latest)	3m 32s
cargo test (macos-latest)	1m 49s
cargo test (ubuntu-latest)	4m 58s
	24m 19s

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