Add optional support for activeTab injection

[fregante]

• Closes Automatically listen to activetab (optionally?) #41
• Part of #4983: Extract and improve activeTab tracking pixiebrix/pixiebrix-extension#4984
• Blocked by Add isContentScriptRegistered #36

Missing

• activeTab support in isContentScriptRegistered or similar,

  • e.g. isContentScriptRegistered(url, tabId?) //=> activeTab still returns dynamic
  • ✅ Done: already possible via sync and explicit possiblyActiveTabs.has(tabId)

• https-only injection logic

  • https://github.com/pixiebrix/pixiebrix-extension/blob/fc3d4a0637fe3440ab1608c0997c9c68fe1fe477/src/background/activeTab.ts#L12:L12
  • specifically I need to check what happens when "active tab" happens on non-http protocols
  • 👌 Added via check in CS. Not really something worth the effort as the check already exists in many places.

• Deal with activeTab reload injections

[fregante]

CI fixed by #48 😮‍💨

[fregante]

This file is ready to be exported to a separate module, likely webext-events

[fregante]

Manually tested:

• via context menu
• via browser action

Broken:

• registered top frame, unregistered inner frame: no op ❌
  • https://fregante.github.io/pixiebrix-testing-ground/Will-receive-CS-calls/Parent?iframe=https://example.com

Edit: cross-origin iframes aren't covered by activeTab