dillon1000/react

[Fizz] External runtime: fix bug in processing existing elements (#26303)

## Summary
Fix bug in how the Fizz external runtime processes existing template
elements.

Bug:
- getElementsByTagName returns a HTMLCollection, which is live.
- while iterating over an HTMLCollection, we call handleNode which
removes nodes

Fix:
- Call Array.from to copy children of `document.body` before processing.
- We could use `querySelectorAll` instead, but that is likely slower due
to reading more nodes.

## How did you test this change?

Did ad-hoc testing on Facebook home page by commenting out the mutation
observer and adding the following.
```javascript
  window.addEventListener('DOMContentLoaded', function () {
    handleExistingNodes(document.body);
  });
```