Make `SubCameraView` into a new primitive shape

[ecoskey]

Step 2 of the camera restructure!

Motivation: In the proposal, I've stolen the SubView name to represent a component that requests a viewport proportional to the size of the render target managed by a compositor. Funnily enough, that means keeping the exact type definition of CameraSubView but using it for a different purpose, which I feel warrants splitting out the idea of a proportional "sub-rectangle" into its own type.

• Renamed SubCameraView to SubRect and move to bevy_render::primitives (bikeshedding badly needed for the name)
• added several utility methods. These mostly consume Self, with the idea being that most of the "frontend" components in the proposal are immutable.
• renamed Camera::sub_view to Camera::crop and edited docs. I think this name better reflects its purpose of limiting the extents the camera actually renders to, and avoids conflicting with future alternate usage of "SubView"

Testing

• Tested examples. They're currently broken lol

TODO:

• [x] fix examples
• [x] ~~fix SubRect::inset_by~~ (omitted from this pr, it's non-trivial I think)
• [x] write migration guide

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