[w3c/gamepad] Add trackpad button support (Issue #191)

The current W3C Gamepad specification does not include mappings for the trackpad button on Sony DualShock and DualSense controllers. This omission leads to inconsistencies across browsers, as observed in WebKit (Safari) and Chromium (Chrome).

### Suggested Update:
1. **Standardize Trackpad Button Mapping:** Introduce a standard mapping for the trackpad button, possibly as button 17, to align with Chromium's existing implementation.
2. **Uniform Button Array Size:** Ensure a 17-element button array for all gamepad controllers to maintain consistency and aid in fingerprinting mitigation.

### Additional Information:
This inconsistency was noted in WebKit bug report [266293](https://bugs.webkit.org/show_bug.cgi?id=266293), with a test conducted on gamepad-tester.com showing different behaviors in Safari Technology Preview and Chrome.

### Impact:
Addressing this will enhance cross-browser compatibility and user experience for gamepad users, especially those using Sony's controllers.

**References:**
- WebKit Bug Report: [266293 – Gamepad: Map DualShock/DualSense Trackpad Button](https://bugs.webkit.org/show_bug.cgi?id=266293)


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