Add custom ansi colors and CSS variables for them (#25546)

Add custom ansi colors and CSS variables for them (#25546)

Use our existing color palette to map to the 16 basic ansi colors. This
is backwards-compatible because it aliases the existing color names.

Side note: I think the colors in `console.css` for console file
rendering are incomplete, but fixing those is out of scope here imo.

Before and after:

<img width="542" alt="Screenshot 2023-06-28 at 00 26 12"
src="https://github.com/go-gitea/gitea/assets/115237/86d41884-bc47-4e85-8aec-621eb7320f0b">

<img width="546" alt="Screenshot 2023-06-28 at 00 28 24"
src="https://github.com/go-gitea/gitea/assets/115237/39fa3b37-d49e-49b1-b6bc-390ac8ca24b2">

---------

Co-authored-by: Giteabot <teabot@gitea.io>