fix(ext/web): Copy EventTarget list before dispatch (#19360)

Related issue: https://github.com/denoland/deno/issues/19358.

This is a regression that seems to have been introduced in
https://github.com/denoland/deno/pull/18905. It looks to have been a
performance optimization.

The issue is probably easiest described with some code:
```ts
const target = new EventTarget();
const event = new Event("foo");
target.addEventListener("foo", () => {
  console.log('base');
  target.addEventListener("foo", () => {
    console.log('nested');
  });
});
target.dispatchEvent(event);
```
Essentially, the second event listener is being attached while the `foo`
event is still being dispatched. It should then not fire that second
event listener, but Deno currently does.

cli/tests/unit/event_target_test.ts

@@ -245,6 +245,20 @@ Deno.test(function eventTargetDispatchShouldSetTargetInListener() {
   assertEquals(called, true);
 });
 
+Deno.test(function eventTargetDispatchShouldFireCurrentListenersOnly() {
+  const target = new EventTarget();
+  const event = new Event("foo");
+  let callCount = 0;
+  target.addEventListener("foo", () => {
+    ++callCount;
+    target.addEventListener("foo", () => {
+      ++callCount;
+    });
+  });
+  target.dispatchEvent(event);
+  assertEquals(callCount, 1);
+});
+
 Deno.test(function eventTargetAddEventListenerGlobalAbort() {
   return new Promise((resolve) => {
     const c = new AbortController();

ext/web/02_event.js

@@ -737,13 +737,14 @@ function innerInvokeEventListeners(
   }
 
   let handlers = targetListeners[type];
+  const handlersLength = handlers.length;
 
   // Copy event listeners before iterating since the list can be modified during the iteration.
-  if (handlers.length > 1) {
+  if (handlersLength > 1) {
     handlers = ArrayPrototypeSlice(targetListeners[type]);
   }
 
-  for (let i = 0; i < handlers.length; i++) {
+  for (let i = 0; i < handlersLength; i++) {
     const listener = handlers[i];
 
     let capture, once, passive;