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-ui-core/src/utils/flipperLibImplementation/index.tsx

@@ -59,5 +59,16 @@ export function initializeFlipperLibImplementation(
       );
     },
     DetailsSidebarImplementation: DetailSidebarImpl,
+    settings() {
+      const darkModeState = store.getState().settingsState.darkMode;
+      let isDarkMode = darkModeState === 'dark';
+      if (
+        darkModeState === 'system' &&
+        window.matchMedia('(prefers-color-scheme:dark)').matches
+      ) {
+        isDarkMode = true;
+      }
+      return {isDarkMode};
+    },
   });
 }