* Introduce a Theme struct as a new part of the app's settings
* Store on each Language a ThemeMap, which converts the capture ids
from that language's highlight query into StyleIds, which identify
styles in the current Theme.
* Update `highlighted_chunks` methods to provide StyleIds instead of
capture ids.
* Include it, along with settings in `OpenParams` grouped under a new struct called `AppState`
Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Max Brunsfeld
