Rework loading images from files (#7088)

This PR is a follow-up to #7084, where I noted that I wasn't satisfied
with using `SharedUri` to represent both URIs and paths on the local
filesystem:

> I'm still not entirely happy with this naming, as the file paths that
we can store in here are not _really_ URIs, as they are lacking a
protocol.
>
> I want to explore changing `SharedUri` / `SharedUrl` back to alway
storing a URL and treat local filepaths differently, as it seems we're
conflating two different concerns under the same umbrella, at the
moment.

`SharedUri` has now been reverted to just containing a `SharedString`
with a URI.

`ImageSource` now has a new `File` variant that is used to load an image
from a `PathBuf`.

Release Notes:

- N/A