Make intervals easier and faster, add window.setInterval and clearInterval

When the browser microtask was added, zig-specific timeout functions were
added to the loop. This was necessary for two reasons:
1 - The existing functions were JS specific
2 - We wanted a different reset counter for JS and Zig

Like we did in https://github.com/lightpanda-io/browser/pull/577, the loop is
now JS-agnostic. It gets a Zig callback, and the Zig callback can execute JS
(or do whatever). An intrusive node, like with events, is used to minimize
allocations.

Also, because the microtask was recently moved to the page, there is no longer
a need for separate event counters. All timeouts are scoped to the page.

The new timeout callback can now be used to efficiently reschedule a task. This
reuses the IO.completion and Context, avoiding 2 allocations. More importantly
it makes the internal timer_id static for the lifetime of an "interval". This
is important for window.setInterval, where the callback can itself clear the
interval, which we would need to detect in the callback handler to avoid
re-scheduling. With the stable timer_id, the existing cancel mechanism works
as expected.

The loop no longer has a cbk_error. Callback code is expected to try/catch
callbacks (or use callback.tryCall) and handle errors accordingly.

src/server.zig

@@ -1042,9 +1042,6 @@ pub fn run(
     // - JS callbacks events from scripts
     while (true) {
         try loop.io.run_for_ns(10 * std.time.ns_per_ms);
-        if (loop.cbk_error) {
-            log.err("JS error", .{});
-        }
     }
 }