Expose dark mode flag to plugin authors

Summary:
Should be available now with
```
getFlipperLib().settings().isDarkMode
```

Differential Revision: D46685851

fbshipit-source-id: 0777437b7ca06051cc3fd4dc1b794242f453f2b5

desktop/flipper-server-companion/src/initializeFlipperLibImplementation.tsx

@@ -38,5 +38,8 @@ export function initializeFlipperLibImplementation(
       // TODO: Write to stdout/stderr?
       throw new Error('Not supported in headless context');
     },
+    settings() {
+      return {isDarkMode: false};
+    },
   });
 }