feat: treat css as entry

BREAKING CHANGE: until now, a CSS file was always coupled to a JS
entry: `vite()->css('index.js')` and the JS entry would have to import
the CSS file. With this change, the CSS file is an entry itself and can
be loaded by its name: `vite()->css('index.css')`.
Note: for this to work the CSS file has to be added to
`rollupOptions.input` in the `vite.config.js`.

Author: arnoson

lib/Vite.php

@@ -142,7 +142,7 @@ public function js($entry = null, $options = []): ?string {
   public function css($entry = null, array $options = null): ?string {
     return !$this->isDev()
       ? css(
-        $this->assetProd($this->manifestProperty($entry, 'css')[0]),
+        $this->assetProd($this->manifestProperty($entry, 'file')),
         $options
       )
       : null;