Add example using MV3 userScripts API

[Rob--W]

Description

Add example using MV3 userScripts API. And many other aspects of MV3 development, useful for #496.

Motivation

This example is part of the documentation of the userScripts API.

Additional details

• User scripts in Manifest V3 w3c/webextensions#279 (History and links to API proposals)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1875475 ("[meta] Implement userScripts API in MV3")
• https://bugzilla.mozilla.org/show_bug.cgi?id=1917000 ("Implement opt-in to userScripts API")

Related issues and pull requests

[Rob--W]

Note: I put strict_min_version 134 because this is the first version of Firefox where the API landed, behind the extensions.userScripts.mv3.enabled preference. I may update it to 135 or 136 once we ship it by default on release.

[erosman]

Note: "host_permissions" should include "file:///*", as many users (myself included) run personal userScripts on local files.

[Rob--W]

Note: "host_permissions" should include "file:///*", as many users (myself included) run personal userScripts on local files.

This is a demo extension, not a full-fledged user script manager. For it to be a complete user script manager, it would have to define the full GM.* / GM_* APIs, and also have a better UI. That goes beyond demonstrating the extension APIs and is left as an exercise to the reader.

[erosman]

I understand.. the intension was to demonstrate that the MV3 userScripts API supports "file:///*" as well.