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