PurgeCSS extractor needs more default extensions, e.g. 'php' or 'html'

[polarbirke]

Only files matching the configured extenstions are parsed (performance reasons). If we add CSS classes in .html or .php files they will currently always be purged because of the current config:

// conditionally run PurgeCSS if any config (local or global) was found
purgeCss ? $.purgecss({
    content: purgeCssConfig.content,
    extractors: [
        {
            extractor: utilityCssExtractor,
            extensions: ['twig', 'js']
        }
    ],
    safelist: purgeCssConfig.safelist
}) : false