Add custom cursors

[eero-lehtinen]

Objective

• Add custom images as cursors
• Fixes #9557

Solution

• Change cursor type to accommodate both native and image cursors
• I don't really like this solution because I couldn't use Handle<Image> directly. I would need to import bevy_assets and that causes a circular dependency. Alternatively we could use winit's CustomCursor smart pointers, but that seems hard because the event loop is needed to create those and is not easily accessable for users. So now I need to copy around rgba buffers which is sad.
• I use a cache because especially on the web creating cursor images is really slow
• Sorry to #14196 for yoinking, I just wanted to make a quick solution for myself and thought that I should probably share it too.

Testing

• Added example which works fine at least on Linux Wayland (winit side has been tested with all platforms).
• I haven't tested if the url cursor works.

Migration Guide

• CursorIcon is now an enum which can hold custom images in addition to system icons. It's also not Copy anymore.
• Old CursorIcon is renamed to NativeCursorIcon