Unicode escapes

The definition of `UnicodeEscape` in §2.4.5:2 (`fls_5v9gx22g5wPm`) characterises the form of these escapes as follows:

> A UnicodeEscape starts with a `\u{` literal, followed by 1 to 6 instances of a `HexadecimalDigit`, inclusive, followed by a `}` character.

But a unicode escape may also contain underscore characters, anywhere after the first hexadecimal digit.
