chore(sio-client): use babel loose mode when transpiling classes

By default, Babel uses `Object.defineProperty()` when transpiling
classes. We'll now use the loose mode which creates a more terse
output.

|          | before  | after   |
|----------|---------|---------|
| min+gzip | 14.9 KB | 14.6 KB |
| min+br   | 13.4 KB | 13.1 KB |

Reference: https://babeljs.io/docs/babel-plugin-transform-classes

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

@@ -27,7 +27,15 @@ const devBundle = {
     babel({
       babelHelpers: "bundled",
       presets: [["@babel/env"]],
-      plugins: ["@babel/plugin-transform-object-assign"],
+      plugins: [
+        "@babel/plugin-transform-object-assign",
+        [
+          "@babel/plugin-transform-classes",
+          {
+            loose: true,
+          },
+        ],
+      ],
     }),
   ],
 };
@@ -49,7 +57,15 @@ const prodBundle = {
     babel({
       babelHelpers: "bundled",
       presets: [["@babel/env"]],
-      plugins: ["@babel/plugin-transform-object-assign"],
+      plugins: [
+        "@babel/plugin-transform-object-assign",
+        [
+          "@babel/plugin-transform-classes",
+          {
+            loose: true,
+          },
+        ],
+      ],
     }),
   ],
 };