Fix strict TypeScript checks for server

plugin/types/ws.d.ts

@@ -0,0 +1,21 @@
+declare module "ws" {
+  export type RawData = Buffer | ArrayBuffer | Buffer[] | string;
+
+  export class WebSocket {
+    static readonly OPEN: number;
+    readonly readyState: number;
+    send(data: string): void;
+    close(code?: number, reason?: string): void;
+    on(event: "message", listener: (data: RawData) => void): this;
+    on(event: "close", listener: (code: number, reason: Buffer) => void): this;
+    on(event: "error", listener: (error: Error) => void): this;
+  }
+
+  export class WebSocketServer {
+    constructor(options: { host?: string; port: number });
+    on(event: "error", listener: (error: Error) => void): this;
+    on(event: "listening", listener: () => void): this;
+    on(event: "connection", listener: (ws: WebSocket, req: import("http").IncomingMessage) => void): this;
+    close(callback?: () => void): void;
+  }
+}