Instead of using some arbitrary unicode characters to render a task as
completed/not completed, I feel that using an actual checkbox from the
components crate makes it look more polished.

Before:

![image](https://github.com/zed-industries/zed/assets/53836821/700de8f8-2e01-4e03-b237-e3da2971f039)

After:

<img width="883" alt="image"
src="https://github.com/zed-industries/zed/assets/53836821/f63d56c3-bfbb-41c8-b150-8ebf973f75e2">


Release Notes:

- Improved visuals of task lists inside the markdown preview