Tree: add generic icons

Created by: oleggromov

Description

Part of the Tree improvements, it adds icons. Ideally, we would have distinct icons per file type but for now it's either of:

• open/closed folder
• generic document icon

https://user-images.githubusercontent.com/2196347/181752912-a5fb4a1a-dd7b-43c7-983a-7fd0d5b510ca.mov

Test plan

1. Check that tree renders directory and file icons
2. Switch to dark mode and check the colors are correct

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.