Add environment-configurable formatter support

[Kyle-Ye]

Summary

• Add EnvironmentConfigurableFormatter for applying EnvironmentValues to Foundation formatters used by text formatting.
• Configure supported formatter types with environment locale, calendar, and time zone where applicable.
• Gate formatter conformances that are only available on Apple platforms.
• Add DocC comments and Swift Testing coverage for each formatter conformance.

Testing

• swift build
• swift build --target OpenSwiftUICoreTests
• swift test --scratch-path .build-env-formatter-test --disable-experimental-prebuilts --filter 'OpenSwiftUICoreTests.EnvironmentConfigurableFormatterTests'\n- Linux swift build --build-path .build-linux with OPENSWIFTUI_LIB_SWIFT_PATH

[augmentcode]

🤖 Augment PR Summary

Summary: Adds a small internal protocol to make Foundation formatters apply text-related EnvironmentValues before producing output.

Changes:

• Introduces EnvironmentConfigurableFormatter with configure(in:) as a standardized hook for environment-driven formatter configuration.
• Adds conformances for DateFormatter, ISO8601DateFormatter, DateIntervalFormatter, and NumberFormatter to apply locale/calendar/time-zone as appropriate.
• Adds additional conformances for measurement/unit formatters (with MeasurementFormatter/DateComponentsFormatter gated to Darwin where needed).
• Documents the intent and scope of environment-driven configuration in DocC-style comments.
• Adds Swift Testing coverage to validate each formatter type picks up the expected environment values.

Technical Notes: The configuration focuses on properties that directly affect formatted text output (locale/calendar/time zone) and keeps platform availability differences behind conditional compilation.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.92%. Comparing base (5bba281) to head (1520e5f).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #884       +/-   ##
===========================================
+ Coverage   13.79%   26.92%   +13.12%     
===========================================
  Files         639      679       +40     
  Lines       41360    47281     +5921     
===========================================
+ Hits         5706    12730     +7024     
+ Misses      35654    34551     -1103     

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