feat: add new theme amber-newt for delta

[artivilla]

Inspired by GitHub's "dark protanopia and deuteranopia" color-vision friendly set visible at https://github.com/settings/appearance.

git diff

[delta]
    features = amber-newt
    true-color = always
    navigate = true    # use n and N to move between diff sections

git diff (add to in ~/.gitconfig)

[delta]
    features = amber-newt
    true-color = always
    navigate = true    # use n and N to move between diff sections
    side-by-side = true

git blame