project panel: Don't change label color even when file has errors (#20600)

With this PR, Git status is now the only thing that can change an item's
label color. So, the summary of how status colors operate in the project
panel is:

- Item icon color is, by default, never changed, _not_ affected by
either diagnostics or Git status
  - This should become configurable in the near future, though
- However, a little x or triangle icon shows up on top of the file type
icon to display diagnostics status
- Label color is _not_ affected by diagnostics but it _is_ affected by
Git status

This aims to reduce color noise and clarify/simplify how each element is
affected.

Release Notes:

- N/A