Make Global Function explicit.

This is the first in a series of changes to make globals explicit. The ultimate
goal of having explicit Globals is to move away from the global HandleScope and
to explicit HandleScopes.

Currently, we treat globals and locals interchangeably. In fact, for Global ->
Local, we just ptrCast. This works because we have 1 global HandleScope, which
effectively disables V8's GC and thus nothing ever gets moved.

If we're going to introduce explicit HandleScopes, then we need to first have
correct Globals. Specifically, when we want to act on the global, we need to
get the local value, and that will eventually mean making sure there's a
HandleScope.

While adding explicit globals, we're keeping the global HandleScope so that we
can minimize the change. So, given that we still have the global HandleScope
the change is largely two things:
1 - js.Function.persit() returns a js.Function.Global. Types that persist global
   functions must be updated to js.Function.Global.
2 - To turn js.Function.Global -> js.Function, we need to call .local() on it.

The bridge has been updated to support js.Function.Global for both input and
output parameters. Thus, window.setOnLoad can now directly take a
js.Function.Global, and window.getOnLoad can directly return that
js.Function.Global.

src/browser/js/Context.zig

@@ -77,6 +77,7 @@ identity_map: std.AutoHashMapUnmanaged(usize, js.Global(js.Object)) = .empty,
 // the @intFromPtr(js_obj.handle). But v8 can re-use address. Without
 // a reliable way to know if an object has already been persisted,
 // we now simply persist every time persist() is called.
+
 global_values: std.ArrayList(v8.Global) = .empty,
 global_objects: std.ArrayList(v8.Global) = .empty,
 global_modules: std.ArrayList(v8.Global) = .empty,