This PR adds a new `WithRemSize` element to the `ui` crate.

This element can be used to create an element tree that has a different
rem size than the base window.

`WithRemSize` can be nested, allowing for subtrees that have a different
rem size than their parent and their children.

<img width="912" alt="Screenshot 2024-05-16 at 2 25 28 PM"
src="https://github.com/zed-industries/zed/assets/1486634/f599cd9f-c101-496b-93e8-06e570fbf74f">

Release Notes:

- N/A