perf(destroyable): replace splice with swap-and-pop in remove()

`remove()` in `@glimmer/destroyable/index.ts:58` is invoked every time
a destroyable is unassociated from its parent (happens during the
destroy phase, from `removeChildFromParent`). Its previous form:

    let index = collection.indexOf(item);
    collection.splice(index, 1);

does an O(n) shift of all elements after `index`. For a parent with
thousands of children being cleared, the total cost is O(n²).

Swap-with-last-then-pop is O(1) for the splice piece (the indexOf
lookup above it is unchanged):

    let index = collection.indexOf(item);
    let lastIndex = collection.length - 1;
    if (index !== lastIndex) {
      collection[index] = collection[lastIndex] as T;
    }
    collection.pop();

Order is not observable. The only consumer of the collection is
`iterate()` (uses `.forEach` for destructors/parents/children); none
of the callers — destructor firing, parent/child propagation —
assume sibling order. Parent-side removal is also batched via
`scheduleDestroyed`, so a child is never removed from the collection
while the parent is iterating it.

Measured with tracerbench (20-fidelity compare vs origin/main):

| phase                | Δ ms        | Δ %      |
|----------------------|------------:|---------:|
| clearManyItems1End   | **-43 ms**  | **-21.3 %** |
| clearManyItems2End   | **-40 ms**  | **-39.5 %** |
| render1000Items1End  |   -2 ms     |  -2.9 %  |
| (all other 17 phases)| no diff     |          |

90% CIs for the two clear-5000 phases: [-46, -40] ms and [-46, -36] ms
respectively — well outside the regression/noise threshold tracerbench
uses. No regressions on any measured phase.

Author: johanrd

packages/@glimmer/destroyable/index.ts

@@ -67,7 +67,11 @@ function remove<T extends object>(collection: OneOrMany<T>, item: T, message: st
 
   if (isBrandedArray(collection) && collection.length > 1) {
     let index = collection.indexOf(item);
-    collection.splice(index, 1);
+    let lastIndex = collection.length - 1;
+    if (index !== lastIndex) {
+      collection[index] = collection[lastIndex] as T;
+    }
+    collection.pop();
     return collection;
   } else {
     return null;