Support Unicode escape in identifier names

[jevancc]

This Pull Request add the support of Unicode escapes (\uXXXX and \u{ cp }) in identifier names. An example for this is:

let \u0078 = 10;
console.log(x); // x = 10

It changes the following:

• Update identifier lexer
• Throw syntax error when there are Unicode escapes in keyword
• Add True, False, Null to keywords (used only in lexing)
• Add unit tests

Some methods in lexer/cursor.rs are not used after this PR. Instead of removing them from the source code, I kept them for future use. The #[allow(dead_code)] is added to prevent clippy error.

[codecov]

Codecov Report

Merging #1102 (8b50842) into master (038acb4) will decrease coverage by 0.03%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
- Coverage   58.81%   58.77%   -0.04%     
==========================================
  Files         176      176              
  Lines       12467    12501      +34     
==========================================
+ Hits         7332     7347      +15     
- Misses       5135     5154      +19     

Impacted Files	Coverage Δ
boa/src/syntax/lexer/cursor.rs	70.24% <ø> (-0.98%)	⬇️
boa/src/syntax/ast/keyword.rs	47.57% <50.00%> (+0.15%)	⬆️
boa/src/syntax/lexer/mod.rs	66.08% <50.00%> (-1.17%)	⬇️
boa/src/syntax/lexer/identifier.rs	55.38% <54.00%> (-6.16%)	⬇️
boa/src/syntax/lexer/string.rs	66.10% <0.00%> (+1.69%)	⬆️

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[jevancc]

This PR is ready for review. The bug related to Unicode escapes in reserved word as property is mentioned in #1117 and I prefer to address it separately. Let me know if you have any comments and suggestions.

[HalidOdat]

Looks good to me :)

[Razican]

Looks good, just a comment for my understanding on dead code.

[Razican]

Why do we allow dead code here? If it's not being used, it should be removed, right?

Same applies to line 195.

[Razican]

Would it be possible to re-base it to see conformance / benchmark changes?

[Razican]

I ran this locally, and got these results:

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,701	78,701	0
Passed	26,400	26,530	+130
Ignored	15,519	15,519	0
Failed	36,782	36,652	-130
Panics	26	27	+1
Conformance	33.54%	33.71%	+0.17%