feat: serve ESM bundle

Related:

- socketio/socket.io-client@0661564
- socketio/socket.io-client#1198

Author: darrachequesne

client-dist/socket.io.esm.min.js

@@ -238,7 +238,7 @@ export class Server<
     this.clientPathRegex = new RegExp(
       "^" +
         escapedPath +
-        "/socket\\.io(\\.min|\\.msgpack\\.min)?\\.js(\\.map)?(?:\\?|$)"
+        "/socket\\.io(\\.msgpack|\\.esm)?(\\.min)?\\.js(\\.map)?(?:\\?|$)"
     );
     return this;
   }

client-dist/socket.io.esm.min.js.map

@@ -125,6 +125,13 @@ describe("socket.io", () => {
         testSourceMap("socket.io.msgpack.min.js.map")
       );
 
+      it("should serve the ESM bundle", testSource("socket.io.esm.min.js"));
+
+      it(
+        "should serve the source map for the ESM bundle",
+        testSourceMap("socket.io.esm.min.js.map")
+      );
+
       it("should handle 304", (done) => {
         const srv = createServer();
         new Server(srv);