fix: Cache Intl.DateTimeFormat instances for better performance

[TrevorBurnham]

Description

Currently, every call to formatDateLocalized (used in DateInput) allocates new Intl.DateTimeFormat instances, which have significant initialization overhead. This PR adds caching for those formatters so that they're only created the first time formatDateLocalized is called.

How has this been tested?

I've added thorough unit test coverage.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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