Add ES6 \u{...} support for identifiers

[ghost]

TypeScript Version: 2.2.0

Code

\u{41}

Expected behavior:
Cannot find name '\u0041'.

Actual behavior:
Syntax warnings are shown.

Advice
It works in string literals, though. This input example is valid in ES6+, and works properly on browser (run in the browser console).

`\u{41}`

For know, only the basic UnicodeEscapeSequence with zeroes is supported.

\u0041

[DanielRosenwasser]

Looks like a duplicate of #5291. We're accepting PRs though.