[whatwg/dom] Should post-connected steps be able to run multiple times during insertion? (Issue #1291)

### What is the issue with the DOM Standard?

See https://github.com/whatwg/dom/pull/1261#discussion_r1594562487 for context. In short, after https://github.com/whatwg/dom/pull/1261, it is possible to run the post-connected steps multiple times/reentrantly for a single element during an insertion. Should that be allowed? Copied from the aforementioned comment:

Imagine a case where you insert two scripts at the same time, i.e., `document.body.append(script1, script2)`. `script2` has an invalid type and therefore is not executable upon insertion as-is. `script1` does two things:

1. Moves `script2` around in the DOM (thus reentrantly invoking the DOM insertion algorithm, and running `script2`'s post-connected steps for the first time)
2. Removes `script2`'s invalid `type` attribute, making it valid/executable (but not immediately executing it, because `type` attribute mutations do not trigger the script processing model).

At least in Chromium, when `script2`'s post-connected steps run for the second time (as a result of the outermost `document.body.append(script1, script2)` call finally invoking `script2`'s post-connected steps), `script2` will finally execute. Here's the code snippet:

```js
const script1 = document.createElement('script');
script1.innerText = `
  const div = document.querySelector('div');
  div.append(script2);
  script2.removeAttribute('type');
`;

const script2 = document.createElement('script');
script2.id = 'script2';
// Makes the script invalid so that it does not run upon insertion.
script2.type = 'foo';
script2.innerText = 'console.log("grazie");';

document.body.append(script1, script2);
```

Despite being generally structured like Blink here, WebKit does not run `script2` in this scenario, which I think indicates that the post-connected steps do not run the second time for script2. I would love to know why / how WebKit does this, @rniwa do you know? And WDYT @smaug----?

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