Remove more usages of JsStr in favor of just JsString

[hansl]

Move the following to JsString (from JsStr):

• get and get_expect now return JsString. Uses code_point_at() instead of get(usize)
• Move the display types to using a ref to the JsString being displayed. Using a JsString reference instead of a clone makes it clearer that calling string.display() should be temporary (and has a lifetime).
• Move trim to JsString and return JsString directly.
• Have the code points iterator be part of the JsString API and its vtable.
• Add inlines and other small time optimizations, like trim() not slicing twice.
• This results in less conversions from JsStr to JsString. It's more performant because creating a JsString from a slice is as fast (thanks to SliceString) than a JsStr on a slice, but creating a JsString from it is faster now.

The end goal is to get rid of JsStr in its current form.

Benchmarks on my machine:

Benchmark	Main	This PR
Richards	235	255
DeltaBlue	245	254
Crypto	207	219
RayTrace	485	525
EarleyBoyer	594	632
RegExp	87.2	90.4
Splay	789	896
NavierStokes	484	490
Total	322	342

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,598	52,598	0
Passed	49,421	49,421	0
Ignored	2,134	2,134	0
Failed	1,043	1,043	0
Panics	0	0	0
Conformance	93.96%	93.96%	0.00%

[codecov]

Codecov Report

❌ Patch coverage is 51.77305% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.65%. Comparing base (6ddc2b4) to head (4197b7d).
⚠️ Report is 631 commits behind head on main.

Files with missing lines	Patch %	Lines
core/string/src/lib.rs	51.19%	41 Missing ⚠️
core/engine/src/builtins/uri/mod.rs	36.36%	7 Missing ⚠️
core/engine/src/builtins/number/globals.rs	70.58%	5 Missing ⚠️
core/string/src/code_point.rs	0.00%	5 Missing ⚠️
core/engine/src/builtins/intl/segmenter/mod.rs	0.00%	4 Missing ⚠️
core/string/src/vtable/slice.rs	0.00%	2 Missing ⚠️
...ore/engine/src/builtins/intl/segmenter/iterator.rs	0.00%	1 Missing ⚠️
...ore/engine/src/builtins/intl/segmenter/segments.rs	0.00%	1 Missing ⚠️
core/string/src/display.rs	0.00%	1 Missing ⚠️
core/string/src/str.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4578       +/-   ##
===========================================
+ Coverage   47.24%   57.65%   +10.41%     
===========================================
  Files         476      531       +55     
  Lines       46892    58764    +11872     
===========================================
+ Hits        22154    33881    +11727     
- Misses      24738    24883      +145     

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[nekevss]

Looks good to me! 😄