Review usage of webNavigation events with regard to pre-rendered documents

[fregante]

https://developer.chrome.com/blog/extension-instantnav/#transitions

According to this document, onCommitted might fire twice on a single navigation:

1. on a pre-rendered document
2. on the visible document

We should make sure that we ignore documents are not yet visible.

As part of this issue, it might also be a good idea to see if the content scripts are injected in the pre-rendered state.

Triaging this as a bug because PB is definitely tracking pre-rendered documents right now, and it shouldn't

[github-actions]

This issue will be closed in 7 days unless the stale label is removed, or a comment is added to the issue.

[fregante]

This PR moved the onCommitted listener to the page itself:

• #6649: Avoid background messaging to handle AJAX page updates #7030

Now we only use webNavigation listeners to keep track of the current tab's URL in the page editor in a hook, so it's fine if it fires twice with the same URL 🙌

pixiebrix-extension/src/pageEditor/hooks/useCurrentUrl.ts

Lines 48 to 52 in 7b1d780

	const startWatching = once(async () => {
	browser.webNavigation.onCommitted.addListener(onNavigation);
	tabUrl = await getCurrentURL();
	urlChanges.emit(tabUrl);
	});