parley: fix `Cursor::previous_visual` by mirroring `Cursor::next_visual` #290

Job	Run time
cargo clippy (windows-latest)	4m 1s
cargo clippy (android) (armv7-linux-androideabi)	1m 35s
cargo check (msrv) (android) (armv7-linux-androideabi)	53s
cargo clippy (android) (aarch64-linux-android)	1m 33s
cargo check (msrv) (windows-latest)	2m 44s
cargo check (msrv) (android) (aarch64-linux-android)	53s
cargo clippy (android) (x86_64-linux-android)	1m 34s
cargo clippy (macos-latest)	1m 46s
cargo check (msrv) (macos-latest)	59s
cargo check (msrv) (android) (x86_64-linux-android)	53s
cargo clippy (ubuntu-latest)	1m 56s
cargo check (msrv) (ubuntu-latest)	1m 0s
cargo doc (windows-latest)	3m 35s
cargo fmt	25s
cargo doc (macos-latest)	1m 4s
cargo test (windows-latest)	4m 13s
typos	9s
cargo doc (ubuntu-latest)	1m 9s
cargo test (macos-latest)	1m 35s
cargo test (ubuntu-latest)	1m 45s
	33m 42s

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