This PR makes the close button for tabs square.

`IconButton` now accepts a `shape`, and using `IconButtonShape::Square`
will ensure the `IconButton` is square with respect to its contained
icon.

#### Before

<img width="119" alt="Screenshot 2024-01-15 at 10 32 40 AM"
src="https://github.com/zed-industries/zed/assets/1486634/dc806b9b-411f-4cd9-8c10-676d2cbd298b">

#### After

<img width="116" alt="Screenshot 2024-01-15 at 10 32 24 AM"
src="https://github.com/zed-industries/zed/assets/1486634/8b4ef43c-14b6-449f-a235-5d7affd82c4e">

Release Notes:

- Changed the tab close button to be square.