Tighten up spacing in the project panel (#2574)

Following https://github.com/zed-industries/zed/pull/2559 the project
panel entries become pretty wide again. This PR tries to mitigate that
and just make some general improvements to visual density in the project
panel.

- Reduces padding around items
- Removes top margin
- Slightly reduces the height of each item
- Fixes an issue where ignored files had the wrong color chevron

Release Notes:

- Improved density of the project panel and tidied up some visual
issues.