refactor(sio-client): mangle private attributes

|          | before  | after   |
|----------|---------|---------|
| min+gzip | 14.6 KB | 14.3 KB |
| min+br   | 13.1 KB | 12.9 KB |

Reference: https://terser.org/docs/options/#mangle-properties-options

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

@@ -15,7 +15,15 @@ module.exports = {
     file: "./dist/socket.io.esm.min.js",
     format: "esm",
     sourcemap: true,
-    plugins: [terser()],
+    plugins: [
+      terser({
+        mangle: {
+          properties: {
+            regex: /^_/,
+          },
+        },
+      }),
+    ],
     banner,
   },
   plugins: [

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

@@ -47,7 +47,15 @@ const prodBundle = {
     format: "umd",
     name: "io",
     sourcemap: true,
-    plugins: [terser()],
+    plugins: [
+      terser({
+        mangle: {
+          properties: {
+            regex: /^_/,
+          },
+        },
+      }),
+    ],
     banner,
   },
   plugins: [