Ability to reset the timezone

[jasonvarga]

Currently, getLocalTimeZone() caches the timezone. There's a todo to have a way to invalidate it, this PR resolves that.

I was writing timezone dependent tests in our own project and needed a way to fake the user's timezone. Due to the cache this wasn't possible.

This adds:

• A setLocalTimeZone method that allows you to specify a timezone explicitly.
• A resetLocalTimeZone to clear the cache which will naturally resolve itself next time getLocalTimeZone is used.

References discussion #7195

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

This PR only exposes two new methods which aren't used in any components. To test this pull request, just run the tests.

[snowystinger]

Just calling out a key real-life use case from the discussion

This can cause unexpected behaviour if the user happens to change timezone (e.g. they close their laptop in one timezone, go somewhere, open it in another timezone).

Typically, we don't add functionality only for tests, so I want to include that there is a real-life use case as well.

[jasonvarga]

Are you asking me to address that use case in this PR, or did you just want to note the reason?

[snowystinger]

I was just noting it. Do you think the use case is not addressed? I do not mean implement the polling, or whatever mechanism people want to use.

[jasonvarga]

Oh yeah for sure. Someone could set up their own polling mechanism to call the new resetLocalTimeZone method.