First-party parallax backgrounds

## What problem does this solve or what need does it fill?

Parallax backgrounds are a vital part of traditional 2D sidescrolling games like platforms.

The technology required to do this well is well-understood: Bevy should ship with a solution for this to make making 2D games in Bevy nicer.

This problem space is well-explored: we should implement and ship this, much like we do for various PBR features in 3D.

## What solution would you like?

Upstream a working parallax solution like https://crates.io/crates/bevy-parallax

This could either go in `bevy_sprite` (behind a feature-flag) or in its own crate. No strong feelings there.

## What alternative(s) have you considered?

We could also re-implement this from scratch, but I'd want to see clear problems with ecosystem crates first.

## Additional context

Any other information you would like to add such as related previous work,
screenshots, benchmarks, etc.

Start by reviewing the existing bevy-parallax crate and the bevy_sprite crate to understand how a first-party implementation could fit. Compare the ecosystem solution with the proposed feature-flagged or separate-crate options. Done means agreeing on a supported parallax-background design and implementing and shipping it in Bevy.

Written by the indexing model from the issue text.