Rework third-party themes (#3254)

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

crates/theme2/src/theme2.rs

@@ -6,6 +6,7 @@ mod scale;
 mod settings;
 mod syntax;
 mod themes;
+mod user_theme;
 
 use std::sync::Arc;
 
@@ -18,10 +19,12 @@ pub use scale::*;
 pub use settings::*;
 pub use syntax::*;
 pub use themes::*;
+pub use user_theme::*;
 
 use gpui::{AppContext, Hsla, SharedString};
+use serde::Deserialize;
 
-#[derive(Debug, PartialEq, Clone, Copy)]
+#[derive(Debug, PartialEq, Clone, Copy, Deserialize)]
 pub enum Appearance {
     Light,
     Dark,