Waiting for upstream to implement the `UserScripts` API

[ACTCD]

• User scripts in Manifest V3 w3c/webextensions#279

• Bug 271472 Consider adding support for the UserScripts API

Once this API is implemented, our lives will be easier in general, and some issues that are currently unavailable will be solved, such as:

• Better isolation and security brought by the new USER_SCRIPT world
• Real run-at document-start injection timing
• User scripts are not restricted by page CSP policy