gpui: Support loading image from filesystem (#6978)

This PR implements support for loading and displaying images from a
local file using gpui's `img` element.

API Changes:
- Changed `SharedUrl` to `SharedUrl::File`, `SharedUrl::Network`

Usage:
```rust
// load from network
img(SharedUrl::network(...)) // previously img(SharedUrl(...)

// load from filesystem
img(SharedUrl::file(...))
```

This will be useful when implementing markdown image support, because we
need to be able to render images from the filesystem (relative/absolute
path), e.g. when implementing markdown preview #5064.

I also added an example `image` to the gpui crate, let me know if this
is useful. Showcase:
<img width="872" alt="image"
src="https://github.com/zed-industries/zed/assets/53836821/b4310a26-db81-44fa-9a7b-61e7d0ad4349">

**Note**: The example is fetching images from [Lorem
Picsum](https://picsum.photos) ([Github
Repo](https://github.com/DMarby/picsum-photos)), which is a free
resource for fetching images in a specific size. Please let me know if
you're okay with using this in the example.

crates/gpui/src/elements/img.rs

@@ -27,18 +27,6 @@ impl From<SharedUrl> for ImageSource {
     }
 }
 
-impl From<&'static str> for ImageSource {
-    fn from(uri: &'static str) -> Self {
-        Self::Uri(uri.into())
-    }
-}
-
-impl From<String> for ImageSource {
-    fn from(uri: String) -> Self {
-        Self::Uri(uri.into())
-    }
-}
-
 impl From<Arc<ImageData>> for ImageSource {
     fn from(value: Arc<ImageData>) -> Self {
         Self::Data(value)