TODO:

- [ ] Remove slop comments
- [ ] Review APIs in detail

# Objective

- This PR aims to add official mobile API surfaces to GPUI

## Solution

- This PR cross references several internal experiments to come up with
a general cross-platform abstraction for core input mechanisms, but
defers actual implementations to later. These are intended to be a
common base to build off of.

## Testing

- Yes, as mentioned above with external experiments

## Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A