chore(benchmark): add `{ __proto: null }` case

So I am just tired of @magic-akari and @mmis1000 and @jack-works asking me why not `{ __proto: null }`, so I updated the benchmark in the PR. I have enabled `Allow edits by maintainers` so you can update the `README.md` file in my branch before merging this. Also here is my run: ``` NullProtoObj via constructor x 207,586,282 ops/sec ±4.80% (81 runs sampled) Object.create(null) x 54,415,324 ops/sec ±2.01% (89 runs sampled) {} (normal object) x 194,340,713 ops/sec ±5.15% (77 runs sampled) {__proto__:null} x 39,313,923 ops/sec ±2.37% (92 runs sampled) Fastest is NullProtoObj via constructor ``` `{ __proto: null }` is even 40% slower than `Object.create(null)`.