[FIX] reactivity: fix memory leak

The reactive cache should be a WeakMap from targets to another WeakMap
that associates callbacks to target. The second WeakMap was in fact a
map, and typescript did not complain because Map has compatible typing
with WeakMap. This commit fixes that.

src/reactivity.ts

@@ -207,7 +207,7 @@ export function reactive<T extends Target>(
     return reactive(originalTarget, callback);
   }
   if (!reactiveCache.has(target)) {
-    reactiveCache.set(target, new Map());
+    reactiveCache.set(target, new WeakMap());
   }
   const reactivesForTarget = reactiveCache.get(target)!;
   if (!reactivesForTarget.has(callback)) {