reorg: move JS ops implementations to cli/js/ops/, part 2 (#4283)

Following JS ops were moved to separate files in cli/js/ops directory:
- io
- process
- worker_host
- web_worker
- plugins
- timers
- signal
- permissions

cli/js/ops/plugins.ts

@@ -0,0 +1,12 @@
+import { sendSync } from "./dispatch_json.ts";
+
+interface OpenPluginResponse {
+  rid: number;
+  ops: {
+    [name: string]: number;
+  };
+}
+
+export function openPlugin(filename: string): OpenPluginResponse {
+  return sendSync("op_open_plugin", { filename });
+}