Expose a `CursorPosition` resource

## What problem does this solve or what need does it fill?

Knowing where your mouse is is useful and should be ergonomic.

## What solution would you like?

Store and update a `CursorPosition` resource, which reads from the `Window` resource to store an `Option` representing the physical position of the mouse (or other cursor).

## What alternative(s) have you considered?

Compute this manually every time it's needed.

## Additional context

This will be impacted by #4530, which will move where this data is stored.

Better APIs to convert between world space and screen space coordinates will make this functionality even more useful; we should add methods on `MousePosition` to do this once those are established.

Start by inspecting the Window resource and the location where cursor position data is currently stored, then read issue #4530 because it changes that storage. Implement the CursorPosition resource as an Option updated from Window data, and verify that consumers can access the physical mouse or cursor position without recomputing it manually.

Written by the indexing model from the issue text.