Fixes #3124

As per #2458 (comment), we require the add-on ID to be specified in the manifest.

⚠️ This is another breaking change, and since it was confusing, I am going to document a few use cases below.

New MV2 add-on, no ID in the manifest

Add-on ID is generated by AMO, a message is printed to update the manifest.json file:

Generated extension ID: {297f6bbb-7604-41ca-8473-0b685e4ae3ff}.
You must add the following to your manifest:
"browser_specific_settings": {"gecko": {"id": "{297f6bbb-7604-41ca-8473-0b685e4ae3ff}"}}

Add-on submitted successfully.

Existing MV2 add-on, no ID in the manifest

An error is thrown by web-ext:

UsageError: Cannot use previously auto-generated extension ID {297f6bbb-7604-41ca-8473-0b685e4ae3ff} - This extension ID must be specified in the manifest.json file.

New MV2 add-on, ID in the manifest

Add-on submitted successfully.

Existing MV2 add-on, ID in the manifest

Add-on submitted successfully.

New MV3 add-on, no ID in the manifest

An error is thrown by web-ext:

UsageError: An extension ID must be specified in the manifest.json file.

New MV3 add-on, ID in the manifest

Add-on submitted successfully.