Add WNP for Firefox 134beta [fix #15489]

[craigcook]

One-line summary

Adds a what's new page for Firefox 134beta. English only.

Issue / Bugzilla link

15489

Testing

http://localhost:8000/en-US/firefox/134.0beta/whatsnew/

[craigcook]

Marking Do Not Merge because we're waiting for legal sign off on the copy. Also the CTA links to a 404, as that campaign page needs to be created.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.83%. Comparing base (e775d32) to head (706cf1f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15511 +/- ## ========================================== + Coverage 78.82% 78.83% +0.01% ========================================== Files 158 158 Lines 8282 8286 +4 ========================================== + Hits 6528 6532 +4 Misses 1754 1754 ```

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🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[craigcook]

Rebased and switched to the whatsnew_events bundle, but we're still blocked on legal review and publishing the community page.

[craigcook]

We have legal signoff and assurances the community page will go live before release day.