feat: add support for time-zone attribute

[davhdavh]

fixes #292

[Copilot]

Pull Request Overview

Adds support for a time-zone attribute to the <relative-time> element, allowing consumers to specify IANA time zones for formatting. Key changes:

• Introduced tests covering setting, changing, and inheriting time-zone.
• Implemented a timeZone getter, observed attribute, and integrated into Intl.DateTimeFormat options.
• Updated README with documentation for the new time-zone attribute.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/relative-time.js	New test suite for time-zone behavior (setting, updating, inheriting)
src/relative-time-element.ts	Added timeZone getter, included 'time-zone' in observedAttributes, and passed timeZone to formatters
README.md	Added documentation entry for the time-zone attribute

Comments suppressed due to low confidence (2)

test/relative-time.js:2590

• [nitpick] The suite name uses camelCase [timeZone], but the attribute is hyphenated time-zone. Rename to [time-zone] for consistency with attribute naming.

  suite('[timeZone]', function () {

test/relative-time.js:2630

• Relying on the system default time zone makes this assertion environment-dependent and potentially flaky. Consider mocking the default time zone or explicitly setting a fallback for consistent test results.

      assert.equal(el.shadowRoot.textContent, 'Wed, Jan 1, 2020, 4:00:00 PM')

[Copilot]

Grammar: change “browsers default” to “browser's default” to fix the possessive form.

Suggested change

	If the individual element does not have a `time-zone` attribute then it will traverse upwards in the tree to find the closest element that does, or default the `time-zone` to the browsers default.
	If the individual element does not have a `time-zone` attribute then it will traverse upwards in the tree to find the closest element that does, or default the `time-zone` to the browser's default.

[TylerJDev]

Hey @davhdavh! 👋

Thanks so much for the PR! We'll aim to take a look at it and review it this week! 😁