chore(sio-client): restore the debug package in the dev bundle

The debug package was not included anymore in the dev bundle since the
migration from webpack to rollup ([1]) in version 4.0.0.

[1]: socketio/socket.io-client@0661564

Author: darrachequesne

packages/socket.io-client/support/rollup.config.umd.js

@@ -10,25 +10,15 @@ const banner = `/*!
  * Released under the MIT License.
  */`;
 
-module.exports = {
-  input: "./build/esm/browser-entrypoint.js",
-  output: [
-    {
-      file: "./dist/socket.io.js",
-      format: "umd",
-      name: "io",
-      sourcemap: true,
-      banner,
-    },
-    {
-      file: "./dist/socket.io.min.js",
-      format: "umd",
-      name: "io",
-      sourcemap: true,
-      plugins: [terser()],
-      banner,
-    },
-  ],
+const devBundle = {
+  input: "./build/esm-debug/browser-entrypoint.js",
+  output: {
+    file: "./dist/socket.io.js",
+    format: "umd",
+    name: "io",
+    sourcemap: true,
+    banner,
+  },
   plugins: [
     nodeResolve({
       browser: true,
@@ -40,4 +30,28 @@ module.exports = {
       plugins: ["@babel/plugin-transform-object-assign"],
     }),
   ],
+}
+
+const prodBundle = {
+  input: "./build/esm/browser-entrypoint.js",
+  output: {
+    file: "./dist/socket.io.min.js",
+    format: "umd",
+    name: "io",
+    sourcemap: true,
+    plugins: [terser()],
+    banner,
+  },
+  plugins: [
+    nodeResolve({
+      browser: true,
+    }),
+    babel({
+      babelHelpers: "bundled",
+      presets: [["@babel/env"]],
+      plugins: ["@babel/plugin-transform-object-assign"],
+    }),
+  ],
 };
+
+module.exports = [devBundle, prodBundle];

packages/socket.io-client/support/rollup.config.umd.msgpack.js

@@ -1,13 +1,15 @@
-const base = require("./rollup.config.umd.js");
+const base = require("./rollup.config.umd.js")[1];
 const alias = require("@rollup/plugin-alias");
+const commonjs = require("@rollup/plugin-commonjs");
 
 module.exports = {
   ...base,
   output: {
-    ...base.output[1],
+    ...base.output,
     file: "./dist/socket.io.msgpack.min.js",
   },
   plugins: [
+    commonjs(),
     alias({
       entries: [
         {