Add toolbar spacing and alignment improvements (#22771)

Tackles some of the points here:
https://github.com/zed-industries/zed/issues/22673. However, this is not
doing anything yet to treat misalignment when with odd-number UI font
sizes. Here are some screenshots with a theme that makes easier to spot
them. It's subtle:

| Before | After |
|--------|--------|
| <img width="1313" alt="Screenshot 2025-01-07 at 10 23 31 AM"
src="https://github.com/user-attachments/assets/fdf125a7-ef1c-4368-aea8-579f916b9c34"
/> | <img width="1313" alt="Screenshot 2025-01-07 at 10 26 11 AM"
src="https://github.com/user-attachments/assets/9728fd47-3c17-4c42-9cf6-11083eb32980"
/> |
| <img width="1313" alt="Screenshot 2025-01-07 at 10 23 36 AM"
src="https://github.com/user-attachments/assets/dc2010e9-4ae4-451c-afd1-6bd13750dc66"
/> | <img width="1313" alt="Screenshot 2025-01-07 at 10 26 08 AM"
src="https://github.com/user-attachments/assets/a71ef2ef-3ac7-4b0a-8d50-1c3c4f17d5cb"
/> |

Release Notes:

- N/A