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/user_theme.rs

@@ -0,0 +1,25 @@
+use refineable::Refineable;
+use serde::Deserialize;
+
+use crate::{Appearance, ThemeColors, ThemeColorsRefinement};
+
+#[derive(Deserialize)]
+pub struct UserThemeFamily {
+    pub name: String,
+    pub author: String,
+    pub themes: Vec<UserTheme>,
+}
+
+#[derive(Deserialize)]
+pub struct UserTheme {
+    pub name: String,
+    pub appearance: Appearance,
+    pub styles: UserThemeStylesRefinement,
+}
+
+#[derive(Refineable, Clone)]
+#[refineable(deserialize)]
+pub struct UserThemeStyles {
+    #[refineable]
+    pub colors: ThemeColors,
+}