ui: Render disclosures with `IconButtonShape::Square` (#13004)

This PR adjusts the `Disclosure` component to render using
`IconButtonShape::Square`.

This tightens up the hover styles so they aren't quite so massive.

### Before

<img width="116" alt="Screenshot 2024-06-13 at 3 22 43 PM"
src="https://github.com/zed-industries/zed/assets/1486634/dea8f8fb-a041-4aa7-89be-0cd2d7889955">

### After

<img width="113" alt="Screenshot 2024-06-13 at 3 23 02 PM"
src="https://github.com/zed-industries/zed/assets/1486634/7682cce6-4f83-4f3e-b91f-3023849bd314">

Release Notes:

- Tweaked the style of disclosure controls throughout the UI.