Moved back pyscript.core where it belongs (#1622)

pyscript.core/rollup/core-css.config.js

@@ -0,0 +1,20 @@
+import postcss from "rollup-plugin-postcss";
+
+export default {
+    input: "./src/core.css",
+    plugins: [
+        postcss({
+            extract: true,
+            sourceMap: false,
+            minimize: !process.env.NO_MIN,
+            plugins: [],
+        }),
+    ],
+    output: {
+        file: "./core.css",
+    },
+    onwarn(warning, warn) {
+        if (warning.code === "FILE_NAME_CONFLICT") return;
+        warn(warning);
+    },
+};