This PR reworks the way we define our third-party themes to make them
work as overlays on top of a base theme.
We introduce the concept of a `UserThemeFamily` that contains
`UserTheme`s. Rather than being an entire theme definition on their own,
a `UserTheme` just contains optional overrides for the values in a
`Theme`.
When resolving a `UserTheme`, we apply it on top of the base theme. Any
values not overridden in the `UserTheme` will fall back to the `Theme`
defaults.
Right now we are just using `UserTheme` to model third-party themes that
we distribute with the Zed binary. However, this same structure can also
be used to import arbitrary user themes (such as from a theme registry,
or even a theme blob from the settings file).
Release Notes:
- N/A
