Tweak Finalizer callbacks

1 - Finalizer callbacks are now give a *Page parameter. Various types no longer
    need to maintain a reference to *Page just to finalize

2 - EventManager now handles v8_handoff == false cleanup. This is largely
    because of the above change, which would require every:

```
defer if (!event._v8_handoff) event.deinit(false);
```

to be turned into:

```
defer if (!event._v8_handoff) event.deinit(false, page);
```

But the caller might not have a page. Besides this, it makes most uses of Event
simpler. But, in some cases, it could leave a window where the event doesn't
reach the EventManager to be properly managed (though, we have no such cases
as of now).

src/browser/webapi/net/Fetch.zig

@@ -45,7 +45,7 @@ pub const InitOpts = Request.InitOpts;
 pub fn init(input: Input, options: ?InitOpts, page: *Page) !js.Promise {
     const request = try Request.init(input, options, page);
     const response = try Response.init(null, .{ .status = 0 }, page);
-    errdefer response.deinit(true);
+    errdefer response.deinit(true, page);
 
     const resolver = page.js.local.?.createPromiseResolver();
 
@@ -184,7 +184,7 @@ fn httpErrorCallback(ctx: *anyopaque, err: anyerror) void {
     // clear this. (defer since `self is in the response's arena).
 
     defer if (self._owns_response) {
-        response.deinit(err == error.Abort);
+        response.deinit(err == error.Abort, self._page);
         self._owns_response = false;
     };
 
@@ -205,7 +205,7 @@ fn httpShutdownCallback(ctx: *anyopaque) void {
     if (self._owns_response) {
         var response = self._response;
         response._transfer = null;
-        response.deinit(true);
+        response.deinit(true, self._page);
         // Do not access `self` after this point: the Fetch struct was
         // allocated from response._arena which has been released.
     }