add postcss plugin

package.json

@@ -14,6 +14,7 @@
     "esbuild": "^0.23.0",
     "marked": "^13.0.3",
     "mime-types": "^2.1.35",
+    "postcss": "^8.4.41",
     "preact": "^10.23.1"
   }
 }

src/plugins/postcss.ts

@@ -0,0 +1,22 @@
+import { Plugin } from "..";
+import postcss from "postcss";
+
+export default class PostCSS extends Plugin {
+  build: undefined;
+
+  name = "postcss";
+  rewriteTriggers = ["css"];
+  renameTo = "css";
+  longLasting = false;
+  plugins: postcss.AcceptedPlugin[] = [];
+
+  constructor(plugins: postcss.AcceptedPlugin[]) {
+    super();
+    this.plugins = plugins;
+  }
+
+  async rewriteFile(file: string) {
+    const data = await postcss(this.plugins).process(file, {from: undefined});
+    return data.css;
+  }
+}