Support messaging with the offscreen page

[fungairino]

• The offscreen page’s __getTabData call was throwing an error, causing the correct receiver to fail in responding with the right tabData.
• The offscreen page, similar to the background service worker, lacks a tab, which this library did not handle correctly.

Notes:
See change that was reverted due to this bug: pixiebrix/pixiebrix-extension@2021198

Action item from: https://www.notion.so/pixiebrix/Postmortem-Release-2-1-6-13543b21a253809bab4cd2aace909433?pvs=4

[grahamlangford]

Related: fregante/webext-detect#32

[fregante]

I added support for offscreen document detection to webext-detect:

• Add isOffscreenDocument fregante/webext-detect#32

As mentioned in an email to Todd, the messenger was likely failing because the offscreen document was not being properly detected. Also this line needs isBackground() || isOffscreenDocument()

webext-messenger/source/thisTarget.ts

Line 58 in 94cc22c

isBackground() ? "not-needed" : "needed";