Add `PlainEditor` method to get IME cursor area #731

Job	Run time
cargo clippy (android) (armv7-linux-androideabi)	1m 5s
cargo check (msrv) (windows-latest)	1m 11s
cargo check (msrv) (android) (armv7-linux-androideabi)	22s
cargo clippy (android) (aarch64-linux-android)	1m 49s
cargo clippy (windows-latest)	2m 39s
cargo check (msrv) (macos-latest)	38s
cargo check (msrv) (android) (aarch64-linux-android)	43s
cargo clippy (android) (x86_64-linux-android)	1m 54s
cargo clippy (macos-latest)	1m 10s
cargo check (msrv) (android) (x86_64-linux-android)	42s
cargo clippy (ubuntu-latest)	1m 8s
cargo check (msrv) (ubuntu-latest)	37s
cargo doc (windows-latest)	1m 15s
formatting	24s
cargo test (windows-latest)	2m 17s
cargo doc (macos-latest)	51s
typos	6s
cargo test (macos-latest)	32s
cargo doc (ubuntu-latest)	24s
cargo test (ubuntu-latest)	40s
	20m 27s

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