search: Highlight hovered token [codemirror]

Created by: fkling

This commit adds back highlighting hovered tokens. Unlike with Monaco, highlighting is implemented separately from hover information (there doesn't seem to be another way).

I also refactored the component slightly to store decorated token information in the state so that it can be reused by other extensions.

I also adjusted the font size which should supposedly be smaller but was always bigger for me (Firefox/Chromium on Linux).

Test plan

Open the search page, type query into the search input and hover over tokens.