Add UTF-16 input parsing

[raskad]

We currently have UTF-16 handling at runtime trough our JsString type. But one thing that is missing is support for UTF-16 input handling itself. One case where this is relevant for boa itself it in the handling of eval. Because we only allow UTF-8 input, we have to convert the input of eval to UTF-8 before parsing it.

To solve this, I added a ReadChar trait to the parser crate that allows the parser to handle different input encodings. The parsing itself is done on unicode code points. To make this work I removed all of the parsing that was done on bytes, since that was assuming that we only handle UTF-8 inputs. Most of that work is in 55752f2.

I added a UTF-16 input type. To get some first positive results I also adjusted the regex parsing to work for non UTF-8 inputs. That should give us some eval and regex related tests that pass now.

[codecov]

Codecov Report

Attention: 73 lines in your changes are missing coverage. Please review.

Comparison is base (35df2de) 47.42% compared to head (e81c12b) 47.37%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
core/parser/src/lexer/string.rs	71.66%	17 Missing ⚠️
core/parser/src/lexer/mod.rs	67.85%	9 Missing ⚠️
core/parser/src/lexer/number.rs	80.00%	9 Missing ⚠️
core/parser/src/lexer/regex.rs	67.85%	9 Missing ⚠️
core/parser/src/source/utf16.rs	43.75%	9 Missing ⚠️
core/parser/src/lexer/template.rs	45.45%	6 Missing ⚠️
core/parser/src/lexer/cursor.rs	87.50%	5 Missing ⚠️
core/parser/src/source/utf8.rs	78.26%	5 Missing ⚠️
core/engine/src/module/mod.rs	0.00%	1 Missing ⚠️
core/parser/src/lexer/private_identifier.rs	0.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3538      +/-   ##
==========================================
- Coverage   47.42%   47.37%   -0.05%     
==========================================
  Files         470      472       +2     
  Lines       45690    45643      -47     
==========================================
- Hits        21667    21625      -42     
+ Misses      24023    24018       -5     

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[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,960	95,960	0
Passed	76,534	76,542	+8
Ignored	18,477	18,477	0
Failed	949	941	-8
Panics	0	0	0
Conformance	79.76%	79.76%	+0.01%

Fixed tests (8):

test/language/literals/regexp/S7.8.5_A1.4_T2.js [strict mode] (previously Failed)
test/language/literals/regexp/S7.8.5_A1.4_T2.js (previously Failed)
test/language/literals/regexp/S7.8.5_A2.1_T2.js [strict mode] (previously Failed)
test/language/literals/regexp/S7.8.5_A2.1_T2.js (previously Failed)
test/language/literals/regexp/S7.8.5_A2.4_T2.js [strict mode] (previously Failed)
test/language/literals/regexp/S7.8.5_A2.4_T2.js (previously Failed)
test/language/literals/regexp/S7.8.5_A1.1_T2.js [strict mode] (previously Failed)
test/language/literals/regexp/S7.8.5_A1.1_T2.js (previously Failed)

[jedel1043]

Really great to finally have proper UTF-16 parsing! I just have some nitpicks that don't block merging.

[HalidOdat]

Great work @raskad!

Just a nitpick :)