fix(sio-client): do not mangle the "_placeholder" attribute (bis)

The "_placeholder" attribute is used when sending binary data, and was
incorrectly mangled (converted to a random short property, like "it",
to reduce the bundle size).

Related:

- ca9e994
- #5215

[skip ci]

Author: darrachequesne

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

@@ -20,6 +20,7 @@ module.exports = {
         mangle: {
           properties: {
             regex: /^_/,
+            reserved: ["_placeholder"],
           },
         },
       }),