Add a simple 2D camera controller

[alice-i-cecile]

Problem

With the merging of #20215, we've created the bevy_camera_controller crate, and have kicked it off with a freecam for 3D. We intend to improve this with the addition of a panorbit camera, per #19741.

However, neither of these are well-suited to 2D games, whether they are top-down or side-view. We should have a controller for these applications as well: allowing users to add a quick-and-dirty debug camera, and laying the foundation for support in the editor.

Proposed solution

Add a simple RTS-style panning camera, probably called PanCamera. Control it with WASD, add QE rotation, zoom with mousewheel or +/-.

Like the freecam, this should have its own module, feature flag and dedicated example.

Edge panning should be omited: it's not very useful and can lead to frustrations when trying to manipulate objects at the edge of a deliberately chosen view or when moving the mouse off the viewport to access tools.

[chintanbhatt2]

Instead of having a separate PanCamera module, could the free cam crate be split into having a FreeCam3d and a FreeCam2d? As an end user, that would seem to be more intuitive when spawning a dev camera.

[alice-i-cecile]

I can see the rationale, but I don't think we should be coupling these controllers in this way. Things like settings structs and so on have different needs in 2D vs 3D.

[syszery]

Hi! I'd like to work on this. I'm new to the project, so I might need a bit more time — please let me know if that's a problem.

Looking forward to giving this a try!