Tell BottomSheet delegate when user starts to pan the sheet

[cbowdoin]

Platforms Impacted

• iOS
• visionOS

Description of changes

Add the delegate method bottomSheetStartedPan to BottomSheetController.swift and invoke it when we begin panning the sheet from the users pan gesture.

Binary change

Total increase: 1,312 bytes
Total decrease: 0 bytes

File	Before	After	Delta
Total	32,302,600 bytes	32,303,912 bytes	⚠️ 1,312 bytes

Full breakdown

File	Before	After	Delta
BottomSheetController.o	544,120 bytes	544,904 bytes	⚠️ 784 bytes
BottomCommandingController.o	865,200 bytes	865,464 bytes	⚠️ 264 bytes
FocusRingView.o	849,608 bytes	849,872 bytes	⚠️ 264 bytes

Verification

Tested by hiding the content of the sheet the sheet when the pan gesture starts.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

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