Add type for Hex color

[ximex]

add a type for hex-color strings that is a bit stricter than only string. type HexColor = `#${string}`;

[jaywcjlove]

@ximex I can add this type to the package. I tried using the HexColor type, but it may not be easy to use in the code because many places have done error handling, which causes type errors.

https://github.com/uiwjs/react-color/blob/632d4e9201e26b42ee7d5bfeda407144e9a6e2f3/packages/color-convert/src/index.ts#L335-L336

https://github.com/uiwjs/react-color/blob/632d4e9201e26b42ee7d5bfeda407144e9a6e2f3/packages/color-convert/src/index.ts#L339

[ximex]

is already done here: https://github.com/uiwjs/react-color/pull/155/files#diff-4a1d4aef5f7a97716359b980c22ef670d78d5f779651e57fe94589767051e95bR306