Unify Prose and Textbox with an underlying `TextRegion` #2212

Job	Run time
cargo check (aarch64-android)	22s
cargo check (msrv) (wasm32)	22s
cargo clippy (windows-latest)	8m 37s
cargo check (msrv) (windows-latest)	2m 43s
cargo clippy (wasm32)	1m 11s
cargo clippy (macos-latest)	1m 50s
cargo doc	1m 59s
cargo check (msrv) (macos-latest)	32s
cargo clippy (ubuntu-latest)	2m 5s
cargo check (msrv) (ubuntu-latest)	40s
formatting	24s
Prime LFS Cache	7s
cargo test (wasm32)	25s
typos	5s
cargo test (windows-latest)	4m 48s
cargo test (macos-latest)	2m 31s
cargo test (ubuntu-latest)	4m 47s
	33m 28s

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