gpui: Implement trackpad pinch-to-zoom on macOS

[kognise]

Implements trackpad pinch-to-zoom on macOS. Windows and Linux should be straightforward but I don't have requisite test devices with me right now.

• All behavior tested on macOS. Does not cause any issues with existing scroll handling.
• Follows the same propagation and blocking behavior as scroll events. Documentation has been updated accordingly.
• I considered integrating into the scroll system, similar to what JavaScript does, but using a separate event is both more logical (the events are separate on every platform) and more backwards-compatible.
• The ZoomDelta is temporarily tagged #[non_exhaustive] because implementations on future platforms might handle zoom deltas differently from macOS (e.g. pixel distance instead of a magnification change). Consumers of this API can just have a _ => {} match arm, which worst-case means their apps will continue to not have zoom functionality on Windows/Linux without intervention.

Release Notes:

• N/A

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[kognise]

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