add config overrides for `focus.*` and `linkVisited`

[Barsnes]

These are the only colors now missing from overrides in our config.

linkVisited should be fine to add, and I think it should work like this:

"overrides": {
  "linkVisited": { "light": "HEX", "dark": "HEX" }
}

LinkVisited is done

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Focus is a bit harder, since I think we need to change our tokens. focus.inner and focus.outer is currently only in the semantic layer, but if we want a theme to be able to override these, it needs to move to the primitive and theme layer as well.