Unify Prose and Textbox with an underlying `TextRegion` #2219

Job	Run time
cargo check (aarch64-android)	29s
cargo check (msrv) (wasm32)	23s
cargo check (msrv) (windows-latest)	2m 2s
cargo clippy (windows-latest)	2m 59s
cargo check (msrv) (macos-latest)	33s
cargo clippy (macos-latest)	1m 35s
cargo check (msrv) (ubuntu-latest)	2m 15s
cargo clippy (wasm32)	1m 25s
cargo clippy (ubuntu-latest)	2m 23s
cargo doc	45s
formatting	59s
Prime LFS Cache	4s
cargo test (wasm32)	35s
typos	9s
cargo test (windows-latest)	3m 50s
cargo test (macos-latest)	1m 34s
cargo test (ubuntu-latest)	5m 7s
	27m 7s

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