Print message to emphasize the need to grant host permissions when a MV3 extension is loaded

[Rob--W]

In Firefox, MV3 extensions are not granted host permissions at install time (MDN: host_permissions: Requested permissions and user prompts). This includes patterns in content scripts and host_permissions. To avoid developer confusion, we should emphasize the need to grant the permissions manually, e.g. by recommending to add -u about:addons (and toggling the permissions manually) or through the extension's own UI.

The condition for printing the message is:

• manifest_version > 3
• host_permissions or content_scripts is non-empty.

Suggested implementation:

• Declare an array (to hold the informative messages). E.g. called helpful_messages
• When target is Firefox (condition in source code), and manifestData.manifest_version > 3 and content_scripts or host_permissions are non-empty arrays, append the help text to the helpful_messages array.
• When the browsers have finally launched, call log.info(); for each message (after extensionRunner.run();)