Syntax error when using Unicode escape sequence with curly brackets in a string

[jevancc]

Describe the bug
The behavior of Unicode code point escape (Unicode escape with curly brackets) is different from the spec.

To Reproduce

"\u{AA}"       // syntax error, should pass
"\u{{AA}}"     // pass, should throw syntax error
"\u{{AA}"      // freeze, should throw syntax error
"\u{AA}}"      // freeze, should pass

Expected behavior
The string should be tokenized and handled correctly. The expected behavior can be found in MDN.
Some lexer tests should also be updated.

Build environment (please complete the following information):

• OS: Windows 10
• Version: OS Build 190941.508
• Target triple: x86_64-pc-windows-msvc
• Rustc version: rustc 1.49.0-nightly (91a79fb29 2020-10-07)

Additional context
It should be bugs in string.rs and cursor.rs.

[Razican]

Hi! Is this solved by #826?

[jevancc]

Hi! Is this solved by #826?

No, it's not. Seems to be an independent bug in string.rs so I created this issue for it :)