Move finalizers to pure reference counting

Takes https://github.com/lightpanda-io/browser/pull/2024 a step further and
changes all reference counting to be explicit.

Up until this point, finalizers_callback was seen as a fail-safe to make sure
that instances were released no matter what. It exists because v8 might never
call a finalizer, so we need to keep track of finalizables and finalize them
on behalf of v8. BUT, it was used as more than a fallback for v8...it allowed
us to be lazy and acquireRef's in Zig without a matching releaseRef (1), because
why not, the finalizer_callback will handle it.

This commit redefines finalizer_callbacks as strictly being a fallback for v8.
If v8 calls the finalizer, then the finalizer callback is removed (2) - we lose
our fail-safe. This means that every acquireRef must be matched with a
releaseRef. Everything is explicit now. The most obvious impact of this is
that on Page.deinit, we have to releaseRef every MO, IO and blob held by the
page.

This change removes a number of special-cases to deal with various ownership
patterns. For example, Iterators are now properly reference counted and when their
RC reaches 0, they can safely releaseRef on their list. This also elimites
use-after-free potential when 2 RC objects reference each other. This should
eliminate some WPT crashes (e.g. /editing/run/insertimage.html)

(1) - We were only ever lazy about releaseRef during shutdown, so this change
won't result in more aggressive collection.

(2) Since 1 object can be referenced from 0-N IsolatedWorlds, it would be more
accurate to say that the finalizer callback is removed when all referencing
IsolatedWorld finalize it.

src/lightpanda.zig

@@ -259,9 +259,6 @@ pub fn RC(comptime T: type) type {
                 return;
             }
             value.deinit(session);
-            if (session.finalizer_callbacks.fetchRemove(@intFromPtr(value))) |kv| {
-                session.releaseArena(kv.value.arena);
-            }
         }
 
         pub fn format(self: @This(), writer: *std.Io.Writer) !void {