WS-NA: Temporal Migration (PR 2: Adds Internal Adapter)

[Isabella-Mitchell]

Resolves JIRA: WS-NA - 10% time. See plan

Summary

Converts file to TS

Wraps all Moment.js calls in psammead-timestamp-container behind an internal date adapter object, so future PRs can swap Moment for Temporal one method at a time without touching the exported functions. Also installs the Temporal polyfill so it's available at runtime.

The exported functions (formatDuration, formatUnixTimestamp, localisedMoment) are unchanged. I've raised this as it's own PR to verify no regressions before swapping in Temporal."

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