Keywords with Unicode escapes should throw syntax errors

[Razican]

Describe the bug
Currently, if we use a keyword with an Unicode escape, it will just work, and interpret it as the keyword itself. This is a follow-up from #1117.

To Reproduce
The following JavaScript code reproduces the issue:

let x = f\u{61}lse;

Expected behavior
This should throw a syntax error. Instead, it just assigns the false boolean value to the x variable.

[veera-sivarajan]

Hi, I'd like to work on this.