Add `AlphaColor::from_kelvin` / `OpaqueColor::from_kelvin` #493

Job	Run time
cargo check (msrv) (windows-latest)	41s
cargo clippy (wasm32)	26s
cargo check (msrv) (wasm32)	16s
cargo check (msrv) (macos-latest)	27s
cargo check (msrv) (ubuntu-latest)	18s
cargo clippy (windows-latest)	1m 4s
cargo doc	27s
cargo clippy (macos-latest)	31s
formatting	1m 34s
cargo miri	1m 49s
cargo clippy (ubuntu-latest)	27s
cargo test (windows-latest)	36s
cargo test (wasm32)	20s
cargo test (macos-latest)	23s
typos	1s
cargo test (ubuntu-latest)	18s
	9m 38s

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