Checks if a username is a valid osu username for registering now. Note that older usernames MAY be longer or contain illegal character combinations. Some character combinations are denied by user registration but are not checked for in here (such as admin).

A valid username according to this function is between 3 and 15 characters long, doesn't start or end with spaces (strip them before passing), consists only out of a-z A-Z 0-9 _ [ ] - (SPACE) and doesn't contain any of these character combinations: (double space), [].

@safe @safe pure nothrow @nogc
scope const(char)[] username
size_t maxLen = 15