[WICG/webcomponents] rethrown 'change' events lose `isTrusted` flag (Issue #1016)

When creating a custom checkbox, the requirements for throwing a `click`, `input`, and `change` event can successfully clone all the native `<input type=checkbox>` implementation except for `.isTrusted` on change.

Because the `change` event is not `composed`, it must be rethrown. eg:

````js
this.shadowRoot.onchange = (e) => this.dispatchEvent(new Event(e.type, e));
````

````html
<x-checkbox>
  #shadow-root
  <input type=checkbox>
</x-checkbox>
````

This isn't a bug or an error, but a limitation of WC. Any instance of change references to `<input type=checkbox>` to `<x-checkbox><x-checkbox>` need to have all their code inspected for `isTrusted` and ignore the value.

To note, `input` is `composed`, so these interactions will stay the same.

------------

Use case:

I'm rebuilding custom `<x-checkbox>` and testing them again the WPT tests for `<input type=checkbox>` here: https://wpt.fyi/results/html/semantics/forms/the-input-element/checkbox.html

Satisfying the trusted state is impossible.

------------

Perhaps there can be a mechanic where an event may remain trusted as long as it is dispatched by the host of the shadowRoot. In other words, `this.redispatchEvent(e)` can internally clone the event with the host as the target, keeping all properties the same. I know crossing boundaries is a somewhat of a pain point for events.


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