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/chat_panel.rs

@@ -714,7 +714,7 @@ fn format_timestamp(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use gpui::HighlightStyle;
+    use gpui::{HighlightStyle, SharedUrl};
     use pretty_assertions::assert_eq;
     use rich_text::Highlight;
     use time::{Date, OffsetDateTime, Time, UtcOffset};
@@ -730,7 +730,7 @@ mod tests {
             timestamp: OffsetDateTime::now_utc(),
             sender: Arc::new(client::User {
                 github_login: "fgh".into(),
-                avatar_uri: "avatar_fgh".into(),
+                avatar_uri: SharedUrl::network("avatar_fgh"),
                 id: 103,
             }),
             nonce: 5,