feat: add whenExtensionDisabled to Conditional extender

[imorland]

Follow up to #3759

Adds whenExtensionDisabled condition to the Conditional extender. Enables developers to provide extend items when a given extension is not installed/enabled.

Usage example:

return [
    (new Extend\Conditional)
        ->whenExtensionDisabled('flarum-tags', [
                ...
        ]),
]