Implement WIP Ascii JsString

[HalidOdat]

This fixes #3097

This is a very WIP implementation, but feedback is welcomed :)

This PR encodes strings that can be represented as ASCII as a byte array, instead of u16 array, this cuts ASCII strings size by 2x.

We also no longer need to convert &str to &[u16] with utf16! macro, allowing conversion from &str to JsString. This cleans the places where we need JsString from &str.

This may also have some interesting optimization that could be applied when we know that the string is in the ASCII range.

[github-actions[bot]]

Test262 conformance changes

Test result	main count	PR count	difference
Total	50,731	50,731	0
Passed	42,973	42,973	0
Ignored	1,395	1,395	0
Failed	6,363	6,363	0
Panics	18	18	0
Conformance	84.71%	84.71%	0.00%

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 62.01084% with 631 lines in your changes are missing coverage. Please review.

Project coverage is 44.47%. Comparing base (6ddc2b4) to head (52d192a). Report is 148 commits behind head on main.

:exclamation: Current head 52d192a differs from pull request most recent head bc980e4. Consider uploading reports for the commit bc980e4 to get more accurate results

Files	Patch %	Lines
boa_cli/src/debug/string.rs	0.00%	47 Missing :warning:
boa_engine/src/builtins/string/mod.rs	66.41%	44 Missing :warning:
boa_engine/src/string/mod.rs	79.27%	40 Missing :warning:
boa_engine/src/builtins/temporal/calendar/mod.rs	47.76%	35 Missing :warning:
boa_engine/src/string/slice.rs	65.11%	30 Missing :warning:
boa_engine/src/builtins/intl/collator/mod.rs	10.00%	27 Missing :warning:
boa_engine/src/builtins/regexp/mod.rs	66.66%	27 Missing :warning:
boa_engine/src/string/str.rs	52.08%	23 Missing :warning:
boa_engine/src/builtins/intl/segmenter/mod.rs	15.00%	17 Missing :warning:
boa_engine/src/builtins/intl/plural_rules/mod.rs	15.78%	16 Missing :warning:
... and 75 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3450      +/-   ##
==========================================
- Coverage   47.24%   44.47%   -2.77%     
==========================================
  Files         476      490      +14     
  Lines       46892    50360    +3468     
==========================================
+ Hits        22154    22398     +244     
- Misses      24738    27962    +3224     

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[HalidOdat]

This is ready for review/merge :partying_face:

In terms of performance it's pretty much the same as on main, a very slight regression that doesn't effect the overall score.

I did some analysis on what is the percentage of latin1 and u16 through out the combined.js execution for allocation strings (so static strings are not counted) and found that ~99.63 are latin1 strings, some methods still converts to u16, we could reduce this number even more by not eagerly converting to u16.

latin1: 9459373,
latin1_size: 64732594 (in bytes),
u16: 34755,
u16_size: 4346430 (in bytes),

There was also a reduction on binary size by ~64KB!

Checked locally and there is not difference between main and this PR in conformance, it seems that data repo's test262 data is not being updated?

[jedel1043]

Checked locally and there is not difference between main and this PR in conformance, it seems that data repo's test262 data is not being updated?

That's weird. I checked the output of more recent PRs such as dependabot's and everything looks fine.

[HalidOdat]

That's weird. I checked the output of more recent PRs such as dependabot's and everything looks fine.

I reran the test262 workflow and now it shows the correct conformance! :)