gpui: Add SharedUrl type (#3975)

This PR adds a `SharedUrl` type to GPUI.

It's just like a `SharedString`, but for denoting that the contained
value is a URL.

Mainlined from @nathansobo's GPUI blog post:
https://github.com/zed-industries/zed/pull/3968/files#diff-7ee75937e2daf7dd53f71b17698d8bd6d46993d06928d411781b9bd739b5f231R9-R12

Release Notes:

- N/A

crates/gpui/src/gpui.rs

@@ -18,6 +18,7 @@ mod platform;
 pub mod prelude;
 mod scene;
 mod shared_string;
+mod shared_url;
 mod style;
 mod styled;
 mod subscription;
@@ -67,6 +68,7 @@ pub use refineable::*;
 pub use scene::*;
 use seal::Sealed;
 pub use shared_string::*;
+pub use shared_url::*;
 pub use smol::Timer;
 pub use style::*;
 pub use styled::*;