Move files to exp_legacy

exp_legacy/instance.ts

@@ -0,0 +1,7 @@
+import * as lib from "@clusterio/lib";
+import { BaseInstancePlugin } from "@clusterio/host";
+
+export class InstancePlugin extends BaseInstancePlugin {
+	// This class is empty because an instance plugin must be defined for a module to be injected
+	// This requirement may change in the future to allow for standalone modules
+}