Improve treemap color scheme

There are two parts of this problem: single file analysis, and diff analysis.

Single file analysis:
- we could use a gradient (red for large code size nodes and gradually go down to yellow for smaller code size nodes)
- use meaningful colors to call out problems

Diff analysis:
- Red / green is confusing for users. We could use non red / green colors to show positive and negative changes in the diff, or we could flip the current scheme between red / green and their meanings
- maybe only use a red -> yellow gradient to show code size additions and some neutral color to show other negative changes in the diff

In general, we should try to use color and maybe some icons to call out potential code size problems