Fix bug in useSharedValueMock (#6742)

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## Summary

The API for sharedValue has changed from using .value to get() and set()
methods. As a result, the mock that previously returned an object with a
single value property no longer works in the tests.

## Test plan

This is a PR for the mocks, so it's not necessary for it.

---------

Co-authored-by: Tomasz Żelawski <[email protected]>

packages/react-native-reanimated/src/mock.ts

@@ -50,7 +50,37 @@ const hook = {
   ): EventHandlerProcessed<Event, Context> => NOOP,
   // useHandler: ADD ME IF NEEDED
   useWorkletCallback: ID,
-  useSharedValue: <Value>(init: Value) => ({ value: init }),
+  useSharedValue: <Value>(init: Value) => {
+    const value = { value: init };
+    return new Proxy(value, {
+      get(target, prop) {
+        if (prop === 'value') {
+          return target.value;
+        }
+        if (prop === 'get') {
+          return () => target.value;
+        }
+        if (prop === 'set') {
+          return (newValue: Value | ((currentValue: Value) => Value)) => {
+            if (typeof newValue === 'function') {
+              target.value = (newValue as (currentValue: Value) => Value)(
+                target.value
+              );
+            } else {
+              target.value = newValue;
+            }
+          };
+        }
+      },
+      set(target, prop: string, newValue) {
+        if (prop === 'value') {
+          target.value = newValue;
+          return true;
+        }
+        return false;
+      },
+    });
+  },
   // useReducedMotion: ADD ME IF NEEDED
   useAnimatedStyle: IMMEDIATE_CALLBACK_INVOCATION,
   useAnimatedGestureHandler: NOOP_FACTORY,