Add support for different file types than t3s

[schliesser]

There are two configuration options where I can choose a file type for the preview templates:

1. In fractal.js I can set: fractal.components.set('ext', '.hbs');
2. In my PHP Component I can override the $extension variable from AbstractComponent.php.

But this breaks the fractal preview as the preview file type is hardcoded to .t3s:

fractal-typo3/index.js

Line 189 in f95d702

writeFile(path.join(componentParent, `_${componentName}-preview.t3s`), component.preview);

I would recommend to use the component extension variable in this line:

writeFile(path.join(componentParent, `_${componentName}-preview.${component.extension}`), component.preview);