Add escape sequence that matches an int value (to match non-code points)

[pschiffmann]

The escape sequence \u{hhhhhh} throws a FormatException if hhhhhh is not a Unicode scalar value. Sometimes, a user might want to search for unmatched surrogate halves, or match a non-string integer sequence and only use the regex syntax to construct their scanner.
For these cases, a pattern \x{h+} should be added that matches the specified number, regardless of whether this number lies outside the Unicode scalar value range.