Restore isConfigured function (#4832)

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## Summary

`isConfigured` was an internal check that due it's directory's
`index.ts` having `export * from ./core` was available as a direct
import from `react-native-reanimated`. Therefore we cannot simply remove
it in a minor version - deleting it raised
react-navigation/react-navigation#11498,
`react-navigation` uses this function
[here](https://github.com/react-navigation/react-navigation/blob/24c03924397a6e59aba9f6b74a9c5cb4b939d9e1/packages/react-native-drawer-layout/src/views/Drawer.tsx#L56)
to detect Reanimated 3 since Reanimated 2 didn't have this function.

This PR restores this function, marks it as deprecated and adds a
separate `public` check to detect Reanimated 3.

## Test plan

🚀

## Note

We should probably locate more functions that are exported as a part of
public API but they shouldn't be.

src/reanimated2/core.ts

@@ -30,6 +30,22 @@ export type ReanimatedConsole = Pick<
   'debug' | 'log' | 'warn' | 'info' | 'error'
 >;
 
+/**
+ * @returns `true` in Reanimated 3, doesn't exist in Reanimated 2 or 1
+ */
+export const isReanimated3 = () => true;
+
+// Superseded by check in `/src/threads.ts`.
+// Used by `react-navigation` to detect if using Reanimated 2 or 3.
+/**
+ * @deprecated This function was superseded by other checks.
+ * We keep it here for backward compatibility reasons.
+ * If you need to check if you are using Reanimated 3 or Reanimated 2
+ * please use `isReanimated3` function instead.
+ * @returns `true` in Reanimated 3, doesn't exist in Reanimated 2
+ */
+export const isConfigured = isReanimated3;
+
 // this is for web implementation
 global._WORKLET = false;
 global._log = function (s: string) {