Simplify Icu API

[jedel1043]

Hides BoaProvider to be crate-private, which makes the overall API easier for users.

[codecov]

Codecov Report

Attention: 50 lines in your changes are missing coverage. Please review.

Comparison is base (47351ef) 49.01% compared to head (a9ae59f) 48.99%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
core/engine/src/context/icu.rs	56.52%	10 Missing ⚠️
core/engine/src/builtins/intl/locale/mod.rs	0.00%	8 Missing ⚠️
core/engine/src/context/mod.rs	11.11%	8 Missing ⚠️
core/engine/src/builtins/intl/collator/mod.rs	0.00%	5 Missing ⚠️
core/engine/src/builtins/intl/list_format/mod.rs	0.00%	4 Missing ⚠️
core/engine/src/builtins/intl/locale/utils.rs	73.33%	4 Missing ⚠️
core/engine/src/builtins/intl/segmenter/mod.rs	0.00%	4 Missing ⚠️
core/engine/src/builtins/string/mod.rs	0.00%	4 Missing ⚠️
core/engine/src/builtins/intl/plural_rules/mod.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3503      +/-   ##
==========================================
- Coverage   49.01%   48.99%   -0.02%     
==========================================
  Files         469      469              
  Lines       48186    48192       +6     
==========================================
- Hits        23618    23613       -5     
- Misses      24568    24579      +11     

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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	76,534	0
Ignored	18,477	18,477	0
Failed	949	949	0
Panics	0	0	0
Conformance	79.76%	79.76%	0.00%

[HalidOdat]

Looks good to me! :)

[nekevss]

Looks like it needs a rebase. But everything else looks good to me!