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-plugin-core/src/test-utils/test-utils.tsx

@@ -106,6 +106,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
       },
       downloadFile: createStubFunction(),
     },
+    settings: createStubFunction(),
   };
 }