[FIX] reactivity: don't subscribe to keys when making reactive

When attempting to create a reactive object, we first check if the
target can be made reactive, this is done with Object.toString, which
internally reads the Symbol.toStringTag on the underlying object. When
trying to make a reactive object from another, for example when
reobserving a reactive or when reading a reactive object from the
context of another, this would read the subscribe the original object to
the Symbol.toStringTag property.

This commit fixes that by calling Object.toString on the underlying
target object where applicable.

tests/reactivity.test.ts

@@ -9,7 +9,7 @@ import {
   markRaw,
   toRaw,
 } from "../src";
-import { reactive } from "../src/runtime/reactivity";
+import { reactive, getSubscriptions } from "../src/runtime/reactivity";
 import { batched } from "../src/runtime/utils";
 import {
   makeDeferred,
@@ -1020,6 +1020,32 @@ describe("Reactivity", () => {
     expect(n3).toBe(2);
   });
 
+  test("reactive inside other: reading the inner reactive from outer doesn't affect the inner's subscriptions", async () => {
+    const getObservedKeys = (obj: any) => getSubscriptions(obj).flatMap(({ keys }) => keys);
+    let n1 = 0;
+    let n2 = 0;
+    const innerCb = () => n1++;
+    const outerCb = () => n2++;
+    const inner = createReactive({ a: 1 }, innerCb);
+    const outer = createReactive({ b: inner }, outerCb);
+    expect(n1).toBe(0);
+    expect(n2).toBe(0);
+    expect(getObservedKeys(innerCb)).toEqual([]);
+    expect(getObservedKeys(outerCb)).toEqual([]);
+
+    outer.b.a;
+    expect(getObservedKeys(innerCb)).toEqual([]);
+    expect(getObservedKeys(outerCb)).toEqual(["b", "a"]);
+    expect(n1).toBe(0);
+    expect(n2).toBe(0);
+
+    outer.b.a = 2;
+    expect(getObservedKeys(innerCb)).toEqual([]);
+    expect(getObservedKeys(outerCb)).toEqual([]);
+    expect(n1).toBe(0);
+    expect(n2).toBe(1);
+  });
+
   // test("notification is not done after unregistration", async () => {
   //   let n = 0;
   //   const observer = () => n++;