Add methods for removing handlers for specific Names

[joshnies]

Adds the following methods for removing only the handlers for specific Names:

/**
Remove all handlers for one or more names. Includes both "key down" and "key up" handlers.
*/
public static func removeHandlers(for names: [Name]) {}

/**
Remove all "key down" handlers for one or more names.
*/
public static func removeKeyDownHandlers(for names: [Name]) {}

/**
Remove all "key up" handlers for one or more names.
*/
public static func removeKeyUpHandlers(for names: [Name]) {}

NOTE: I also made legacyKeyDownHandlers and legacyKeyUpHandlers internal for testing purposes.

[sindresorhus]

I'm not sure it makes sense to add these methods as onKeyDown/onKeyUp are legacy and will be deprecated in the future. The recommended API is .events() which does not need this.