MDN get started tutorial examples to manifest V3 (#608)

* MDN get started tutorial examples to manifest V3

* Migrate to Scripting API

* Updated to readmes and code comments

* Apply suggestions to readmes from review

Co-authored-by: Simeon Vincent <svincent@gmail.com>

* Convert choose_beast.js to async

* Apply suggestions from review

Co-authored-by: Simeon Vincent <svincent@gmail.com>

* Updated illustrated API list for beastify

* Apply suggestions from review

Co-authored-by: Rob Wu <rob@robwu.nl>

---------

Co-authored-by: Simeon Vincent <svincent@gmail.com>
Co-authored-by: Rob Wu <rob@robwu.nl>

examples.json

@@ -31,12 +31,12 @@
   {
     "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast.",
     "javascript_apis": [
-      "extension.getURL",
+       "runtime.getURL",
       "runtime.onMessage",
+      "scripting.insertCSS",
+      "scripting.removeCSS",
       "tabs.executeScript",
-      "tabs.insertCSS",
       "tabs.query",
-      "tabs.removeCSS",
       "tabs.sendMessage",
       "tabs.Tab"
     ],