WIP custom cursors

[amytimed]

hello, this PR is a wip draft PR to add custom cursor support to bevy

Objective

• Adds custom cursor support to Bevy
• Fixes #9557

Solution

• Splits CursorIcon into two enum variants: SystemCursor and CustomCursor

Testing

As this is an unfinished draft PR, it has not yet been tested

[SpecificProtagonist]

Does it make sense to flatten SystemCursor into CursorIcon?

Also, we might want CustomCursor to consist of a Handle<Image> + hotspot for easy use with assets, though that means you need bevy_render for custom cursors, so maybe a convenience method that converts the image to Rgba will have to do.

[alice-i-cecile]

Closing in favor of #14284.