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/collab_ui/src/notifications/stories/collab_notification.rs

@@ -1,4 +1,4 @@
-use gpui::prelude::*;
+use gpui::{prelude::*, SharedUrl};
 use story::{StoryContainer, StoryItem, StorySection};
 use ui::prelude::*;
 
@@ -19,7 +19,7 @@ impl Render for CollabNotificationStory {
                 "Incoming Call Notification",
                 window_container(400., 72.).child(
                     CollabNotification::new(
-                        "https://avatars.githubusercontent.com/u/1486634?v=4",
+                        SharedUrl::network("https://avatars.githubusercontent.com/u/1486634?v=4"),
                         Button::new("accept", "Accept"),
                         Button::new("decline", "Decline"),
                     )
@@ -36,7 +36,7 @@ impl Render for CollabNotificationStory {
                 "Project Shared Notification",
                 window_container(400., 72.).child(
                     CollabNotification::new(
-                        "https://avatars.githubusercontent.com/u/1714999?v=4",
+                        SharedUrl::network("https://avatars.githubusercontent.com/u/1714999?v=4"),
                         Button::new("open", "Open"),
                         Button::new("dismiss", "Dismiss"),
                     )