Unify Prose and Textbox with an underlying `TextRegion` #2220

Job	Run time
cargo check (msrv) (wasm32)	28s
cargo check (aarch64-android)	25s
cargo clippy (windows-latest)	3m 29s
cargo check (msrv) (windows-latest)	1m 10s
cargo clippy (macos-latest)	2m 25s
cargo check (msrv) (macos-latest)	36s
cargo clippy (wasm32)	1m 23s
cargo clippy (ubuntu-latest)	2m 41s
cargo doc	43s
cargo check (msrv) (ubuntu-latest)	1m 6s
formatting	58s
cargo test (wasm32)	24s
Prime LFS Cache	6s
typos	6s
cargo test (windows-latest)	4m 9s
cargo test (macos-latest)	1m 31s
cargo test (ubuntu-latest)	3m 47s
	25m 27s

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