Fix style preprocessor return value

It is now compatible with both version before and after 3.30.0

sveltejs/svelte#5584

MelteCompiler.js

@@ -360,8 +360,10 @@ SvelteCompiler = class SvelteCompiler extends CachingCompiler {
         },
         style: async ({ content, attributes }) => {
           if (this.postcss && (!attributes.lang || attributes.lang === 'postcss')) {
+            const { css: code } = await this.postcss.process(content, { from: undefined });
+
             return {
-              code: await this.postcss.process(content, { from: undefined })
+              code
             };
           }
         }