fix(intl): implement percent style for Intl.NumberFormat

[rubys]

Summary

Closes #5246.

• Multiply input by 100 (multiply_pow10(2)) for style: "percent" per ECMA-402 FormatNumeric
• Append percent sign to the formatted output via new format_to_js_string() helper
• Update all three call sites (NumberFormat.format, Number.toLocaleString, BigInt.toLocaleString) to use the new helper

Known limitation

The percent sign is currently appended with a narrow no-break space (U+202F) uniformly across all locales. This is correct for most locales (e.g., French "50 %", German "50 %"), but some locales (e.g., English) conventionally use no space ("50%"). The proper fix requires locale-aware percent formatting from ICU4X (see icu4x#4483). Once PercentFormatter is available upstream, this workaround should be replaced.

Test262 conformance (test/intl402/NumberFormat)

Test result	main	PR	difference
Passed	132	133	+1
Ignored	0	0	0
Failed	121	120	-1
Panics	0	0	0

Fixed tests (1):

• test/prototype/format/percent-formatter.js (previously Failed)

No regressions.

Test plan

• cargo test -p boa_engine --features intl_bundled -- number_format — all 4 tests pass
• cargo test -p boa_engine --features intl_bundled -- to_locale_string — existing tests unaffected
• cargo clippy --all-features --all-targets — no warnings
• cargo fmt -- --check — clean
• boa_tester against test/intl402/NumberFormat — +1 fixed, 0 broken

🤖 Generated with Claude Code

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,732	50,733	+1
Ignored	1,426	1,426	0
Failed	805	804	-1
Panics	0	0	0
Conformance	95.79%	95.79%	+0.00%

Fixed tests (1):

test/intl402/NumberFormat/prototype/format/percent-formatter.js (previously Failed)

Tested main commit: 2437b6704c4315f6ec8f0766bf229fba23e0e61e
Tested PR commit: 64953419b6defdd8553d4e0797011e8463981445
Compare commits: 2437b67...6495341

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.87%. Comparing base (6ddc2b4) to head (6495341).
⚠️ Report is 919 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/bigint/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5257       +/-   ##
===========================================
+ Coverage   47.24%   59.87%   +12.63%     
===========================================
  Files         476      589      +113     
  Lines       46892    63415    +16523     
===========================================
+ Hits        22154    37970    +15816     
- Misses      24738    25445      +707     

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[jedel1043]

As we've mentioned in other PRs, we prefer to wait for the required i18n APIs to be available from ICU4X instead of hacking around its limitations