Start loading new theme JSON format instead of TOML

Replaced remaining extends with javascript object extension.
Moved tokens/core.ts to tokens.ts and massaged the types to make it more
obvious when types don't match up.

Co-authored-by: Nathan Sobo <nathan@zed.dev>

styles/styleTree/workspace.ts

@@ -137,5 +137,10 @@ export default function workspace(theme: Theme) {
       ...text(theme, "mono", "secondary"),
       padding: { left: 6 },
     },
+    disconnectedOverlay: {
+      ...text(theme, "sans", "primary"),
+      color: "#ffffff",
+      background: "#000000aa",
+    },
   };
 }