Show image loading errors to the user

[emilk]

Hi, and thanks for your work on egui_commonmark!

In this PR I've added support for showing image loading errors to the user:

To implement this I added a dependency on the poll-promise crate. An added benefit is that we can also show a loading spinner while an image is being downloaded:

I also changed the Mutex to the one from parking_lot. egui already depends on parking_lot, so it is no added dependency, and it is much more ergonomic (no need for .unwrap() everywhere).

I split the code in a few files - hope that's ok.

[lampsitter]

Awesome! It looks great.
I'm on board with the added dependencies.

The only thing that is a little bit annoying is that poll-promise always pulls in the document-features crate, but it's not a show stopper.

I will approve after the typo fix 😃

[lampsitter]

Thanks!