feat: expose MouseWheel::touch_phase

[chompaa]

Objective

• winit::event::WindowEvent::MouseWheel::phase is not exposed throughbevy_input::mouse::MouseWheel.
• I hit this when mapping bevy mouse wheel events to egui, which expects a touch phase parameter.

Solution

• Add bevy_input::mouse::MouseWheel::phase.
• For completeness, also add bevy_picking::pointer::PointerAction::Scroll::phase and bevy_picking::events::Scroll::phase.

Testing

• cargo run --example mouse_input_events with a touch pad and without.
  • Observed the Started, Moved and Ended phases showing up properly.

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Future Work

The gesture types should also have TouchPhase exposed on them, but doing so turns this PR quite a bit larger, so I've left them out.