Decode webp.ImageConfig natively

Decode webp.ImageConfig natively

This has 2 main benefits:

1. When you only need width/height, we don't need to spin up a WASM runtime.
2. When all processed images are cached in the file cache, we don't need to spin up a WASM runtime.

The above is only really visible for smaller sites, but it's annoying to potentially have a site building in 15ms take e.g. 200ms because of WASM startup time.

Fixes #14371