Exclude discarded tabs; inject tabs progressively

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• For Existing tabs are not connectable after an install or extension update pixiebrix/pixiebrix-extension#6548

Missing:

• Exclude Firefox because it's already the default behavior #1

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There's one last scenario that needs to be covered:

• In tracked tabs, when the document reloads or a new document is loaded

At that point, the native content script is injected, so we should stop tracking it.

However this is rather rare, it requires the tab to autonomously reload/redirect without the user even activating it first.

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Finally merging and releasing. Will open an issue for that