fix: use a terser-compatible representation of the separator

By default, terser (the mangler used by webpack) will replace the '\x1e'
by an unreadable character in the optimized output.

One solution would be to use the 'ascii_only' option:

```
module.exports = {
  optimization: {
    minimize: true,
    minimizer: [
      new TerserPlugin({
        terserOptions: {
          output: {
            ascii_only: true
          }
        }
      })
    ]
  }
};
```

But it would require anyone who tries to bundle the engine.io client to
use this plugin, so we'll rather use String.fromCharCode().

Note: String.fromCharCode(30) === '\u001e' === '\x1e'