Add script-on-click example and related content

Add a tutorial using script-on-click: an example of how to create a cross-browser MV3 extension example that uses a service worker on browsers that support it and an event page on browsers that do not support it (Firefox in particular).

The code sample was part of a presentation that shows this progression:

• background.scripts, works in Firefox (and Safari), code on slides available at script-on-click-1
• background.service_worker, works in Chrome (and Safari), code on slides available at script-on-click-2
• A single cross-browser manifest, works across all browsers, code already linked before at https://github.com/Rob--W/fosdem-2024-ext/tree/main/code-samples/script-on-click

Note on preferred_environment: Neither Firefox nor Safari requires background.preferred_environment, if both scripts and service_worker are specified, both browsers choose scripts. preferred_environment should only be used if the dev prefs to use service_worker where available. This behavior is documented in detail at https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background#browser_support.

...and if you need even more context on the design of the feature and the cross-browser behavior, there is more to read at https://github.com/w3c/webextensions/issues/282MDN Web Docsbackground - Mozilla | MDN Use the background key to include one or more background scripts, a background page, or a Service worker in your extension.