PlainEditor: Add byte selection and navigation operations. #442

Job	Run time
cargo check (msrv) (windows-latest)	1m 25s
cargo clippy (android) (armv7-linux-androideabi)	1m 39s
cargo check (msrv) (android) (armv7-linux-androideabi)	53s
cargo check (msrv) (macos-latest)	28s
cargo clippy (android) (aarch64-linux-android)	1m 43s
cargo check (msrv) (android) (aarch64-linux-android)	24s
cargo check (msrv) (ubuntu-latest)	24s
cargo clippy (windows-latest)	1m 45s
cargo clippy (android) (x86_64-linux-android)	1m 35s
cargo check (msrv) (android) (x86_64-linux-android)	47s
cargo clippy (macos-latest)	49s
cargo clippy (ubuntu-latest)	1m 0s
cargo doc (windows-latest)	1m 43s
cargo fmt	21s
cargo doc (macos-latest)	28s
cargo doc (ubuntu-latest)	21s
cargo test (windows-latest)	1m 20s
typos	7s
cargo test (macos-latest)	53s
cargo test (ubuntu-latest)	29s
	18m 34s

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