Regexp decimal escape handling still not correct

André Bargull's report shows that decimal escape handling in RegExpScanner is still not fully compatible to other JS engines:

/\471/.test("\x271")
=> should return true.
This means that when parsing backreferences that could be converted to decimal escapes later on we must consider the max values for octal escapes which is 255. 47 is within that limit while 471 is not.

/\08/.test("\x008")
=> should return true instead of throwing an exception.
This means when encountering something starting with \0 we should check whether the next character is an octal digit, not a decimal digit.

/\8/.test("\\8")
=> should return true instead of false.
This means when a illegal backreference can't be converted to an octal escape it should be treated as literal backslash sequence.