Simplify module and type naming scheme for library consumers

For the next major version, we may consider a new naming scheme that is simpler for downstream consumers, e.g.

- Modules
- `Graphics` -> `GraphicsBase`
- `PlatformGraphics` -> `Graphics` (re-exporting everything needed, including `Utils`)
- Types
- `Image` (+ `BufferedImage`) -> `ImageProtocol`
- `PlatformImage` -> `Image`

Ideally, we would want parity between the different backends and polyfill as needed, so that they can be used interchangeably. It should not matter whether a `CoreGraphicsImage` or a `CairoImage` is used and portable code should not have to deal with this distinction, i.e. `Image` should always refer to the "one true" image type.

Perhaps introducing a wrapper layer (e.g. have `Image` and `GraphicsContext` be shared types that internally uses a platform-specific backends) would be worth looking into, to avoid accidentally leaking platform-specific implementation details if the platform-specific types implement something not declared in the protocol. This might require renaming the protocol for clarity (e.g. `BackingImage`, `ImageBackend` or similar).

Start by inventorying the existing Graphics, PlatformGraphics, Image, BufferedImage, PlatformImage, and backend types mentioned in the issue. Research whether a shared wrapper or renamed protocol can provide backend parity without leaking platform details; done requires an agreed naming scheme and portable Image and graphics types across CoreGraphics and Cairo backends.

Written by the indexing model from the issue text.