fix: add setDateFnsTzModule for Vite compatibility

[PurkkaKoodari]

Description

Linked issue: #6204

Problem
In some bundlers including Vite, dynamic require() or require() in ES modules doesn't exist. This means the timeZone prop will never work in those modes.

Changes
Adds a new exported function setDateFnsTzModule() that allows users to provide the date-fns-tz module explicitly. This works around the dynamic require() that doesn't work in Vite and potentially other bundlers (see #6204).

While this involves some new global state, one would reasonably expect to set this once within an app. (Problems might arise if an intermediate library attempts to sets this to a version different from the one the root app uses, but that doesn't sound very stable with the dynamic require either.)

I might consider adding a if (dateFnsTzLoadAttempted) throw ... to make sure the function is only called once; however, I didn't commit that addition yet.

Screenshots

N/A

To reviewers

Important: In fact, presumably after #6155, I can't get timeZone to work even with Webpack and the hello-world application in the repo. I think this might need at least a docs update if this is broken in Webpack as well, hence keeping as draft now. (Also mentioned in #6182 (comment))

The diff was mostly written by Claude, but I've reviewed it thoroughly, and the test coverage seems sufficient to me.

Other potential paths for implementation would be significantly harder:

• Allow callers to provide dateFnsTz as a prop - would require significant refactoring to date_utils.ts as it now uses global state.
• Use import instead of require in ES modules - also causes significant challenges, as import is asynchronous, and now one couldn't necessarily render while the promise is pending.

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.