perfer-character-class: Reorder alternatives if it's safe

[RunDevelopment]

If the only thing preventing two one-character alternatives from being merged is an alternative that cannot start with either characters (from the one-character alternatives), then it's safe to reorder the alternatives and merge the one-character alternatives.

Example:
(?:a|foo|b) == (?:a|b|foo) == (?:[ab]|foo)

I already added a util method that can detect the first character of an alternative, so this should be easy to implement.

[RunDevelopment]

Closed by 6aae427