Improve unicode escape in regex

[HenryRLee]

This PR supports regex Unicode escape in curly brackets format, provided that the 'u' flag presents.

For example:

let x = /\u{01ac}/u;

Also, it extends Unicode up to 0x10FFFF. For example:

let x = /\u{10FFFF}/u;

I believe this PR also fixes issue #3563.

[ctjlewis]

Nice work, Henry! Code LGTM.

I actually submitted a couple of Unicode logic PRs myself to try to learn more about the standard - I'd love if you could review one of them for me. See #3645, #3647.

[HenryRLee]

Thanks @christiantjl.

I saw your PRs. #3645 looks good to me. When I have time, I will verify the correctness of the Unicode categories in #3647.