ffmpeg: allow import in nodejs (#577)

* Add empty node module

* Switch to empty.mts

packages/ffmpeg/src/empty.mts

@@ -0,0 +1,7 @@
+// File to be imported in node enviroments
+
+export class FFmpeg {
+  constructor() {
+    throw new Error("ffmpeg.wasm does not support nodejs");
+  }
+}