Manifest V3 Beastify

MV3 version of the classic WebExtension example "Beastify".

What it does

The extension includes:

a browser action with a popup including HTML, CSS, and JS
- CSS injection and script execution now uses the scripting API instead of the tab API
a content script
three images, each of a different beast, packaged as web accessible resources
a manifest which:
- uses the new MV3 syntax as indicated for the key web_accessible_resources.
- has replaced the 'browser_action' key for action.
- includes permission for the scripting WebExtension API which was previously not used.

When the user clicks the browser action button, the popup is shown, enabling the user to choose one of three beasts.

When it is shown, the popup injects a content script into the current page.

When the user chooses a beast, the extension sends the content script a message containing the name of the chosen beast.

When the content script receives this message, it replaces the current page content with an image of the chosen beast.

When the user clicks the reset button, the page reloads, and reverts to its original form.

Note that:

if the user reloads the tab, or switches tabs, while the popup is open, then the popup won't be able to beastify the page any more (because the content script was injected into the original tab).
by default scripting.executeScript() injects the script only when the web page and its resources have finished loading. This means that clicks in the popup will have no effect until the page has finished loading. Immediate execution requires to set the optional parameter ´injectImmediately´ to true.
it's not possible to inject content scripts into certain pages, including privileged browser pages like "about:debugging" and the addons.mozilla.org website from a Mozilla browser. If the user clicks the beastify icon when such a page is loaded into the active tab, the popup displays an error message.