Support for asset collections and arrays

[A----]

This PR adds support for collections (named resources) of assets and arrays.

I did not really get what the previous code was supposed to do when the first parameter was an array. It wouldn't work since the actual call on Grav's asset instance would be addAdd ( https://github.com/getgrav/grav-plugin-shortcode-core/blob/develop/shortcode-core.php#L194 ).

While previously I had to do this:

            $this->shortcode->addAssets('js', 'plugin://a-plugin/assets/a-js.js');
            $this->shortcode->addAssets('css', 'plugin://a-plugin/assets/a-css.css');

With no possibility of overloading those assets. I can now do:

// in a plugin
    public function onAssetsInitialized()
    {
        $assets = $this->grav['assets'];

        $assets->registerCollection('a-collection', [
            'plugin://a-plugin/assets/a-js.js',
            'plugin://a-plugin/assets/a-css.css'
        ]);
    }

// in the shortcode
            $this->shortcode->addAssets('a-collection');

It also fixes the possibility of adding assets by array or string, similar to what Grav's Asset add() method does ( https://learn.getgrav.org/16/api#class-grav-common-assets )