refactor: prefix private attributes to allow property mangling

|          | before  | after   |
|----------|---------|---------|
| min      | 27.3 KB | 26.1 KB |
| min+gzip | 8.8 KB  | 8.7 KB  |
| min+br   | 8.0 KB  | 7.8 KB  |

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