feat(Intl): add initial Intl.DisplayNames implementation

[Lokesh0224]

Summary

This PR adds an initial implementation of Intl.DisplayNames, as part of the work tracked in #3710.

---

What's implemented

• Constructor

  • Validates NewTarget per ECMA-402 §12.1.1 (step 1)
  • Creates the object using the engine’s equivalent of OrdinaryCreateFromConstructor (step 2)
  • Documents steps 3–23 inline for future implementation

• supportedLocalesOf

  • Implemented per §12.2.2

• Options infrastructure

  • DisplayNamesType enum covering all six types:
    language, region, script, currency, calendar, dateTimeField
  • OptionType implementations:
    • Style → narrow | short | long
    • Fallback → code | none
    • LanguageDisplay → dialect | standard
  • Note: these rely on icu_experimental and may change as icu4x#3913 evolves

• Tests

  • Unit tests for all valid DisplayNamesType parsing cases

---

Not yet implemented

• of() — §12.3.3

  • Code canonicalization (CanonicalCodeForDisplayNames, §12.5.1)
  • Field lookup
  • Fallback handling

• resolvedOptions() — §12.3.2

  • Population of properties from internal slots (Table 18)

• Internal slot storage

  • Mapping of spec internal slots to struct fields:
    [[Locale]], [[Style]], [[Type]], [[Fallback]],
    [[LanguageDisplay]], [[Fields]]

---

Notes

• LocaleDisplayNamesV1 is used as a temporary stand-in for Service::LangMarker to support supportedLocalesOf locale availability checks.
• Intl.DisplayNames supports multiple categories (language, region, script, currency, calendar, dateTimeField), each backed by different ICU providers. Once formatting is implemented, dispatch will occur at runtime in of() based on the type option.
• The upstream icu4x#3913 is actively progressing, and the remaining data marker design (attribute-based vs. per-type markers, ZeroTrie-backed multi-attribute loading) is targeted for icu4x 2.3. The Service::LangMarker choice will need revisiting once that design stabilizes.
• Constructor steps 3–23 are intentionally deferred due to ongoing upstream discussions in icu4x#3913, particularly around pattern loading (nested parentheses), find_longest_matching_subtag behavior, and multi-attribute data loading.
• Style, Fallback, and LanguageDisplay in options.rs are from icu_experimental and subject to change as the upstream API stabilizes. The LanguageDisplay groundwork already aligns with the "support dialect option" item in icu4x#3913.

[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/DisplayNames/options-languagedisplay-valid.js (previously Failed)

Tested main commit: 8fcbfcbb8b9e70fe878d28376a6496eaa57a70d7
Tested PR commit: 7a6c2e471e6c625489bdc9afa9df966cf70017f5
Compare commits: 8fcbfcb...7a6c2e4

[codecov]

Codecov Report

❌ Patch coverage is 32.05128% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.70%. Comparing base (6ddc2b4) to head (7a6c2e4).
⚠️ Report is 918 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/intl/display_names/mod.rs	22.22%	28 Missing ⚠️
.../engine/src/builtins/intl/display_names/options.rs	24.24%	25 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5249       +/-   ##
===========================================
+ Coverage   47.24%   59.70%   +12.46%     
===========================================
  Files         476      591      +115     
  Lines       46892    63486    +16594     
===========================================
+ Hits        22154    37905    +15751     
- Misses      24738    25581      +843     

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