Add benchmark for node 20

Running the benchmark locally on node 20 shows the regression from >node 14 is fixed.

```
$ yarn start
yarn run v1.22.19
$ node index.js
esm named import x 192,411,982 ops/sec ±0.54% (92 runs sampled)
cjs named import x 197,725,482 ops/sec ±0.48% (98 runs sampled)
esm default import x 194,759,396 ops/sec ±0.62% (98 runs sampled)
cjs default import x 198,171,349 ops/sec ±0.38% (95 runs sampled)
esm namespace import x 1,305,104,381 ops/sec ±0.38% (96 runs sampled)
cjs namespace import x 1,298,098,129 ops/sec ±0.41% (97 runs sampled)
esm const binding of namespace import x 1,296,285,719 ops/sec ±0.45% (95 runs sampled)
cjs const binding of namespace import x 1,295,945,808 ops/sec ±0.50% (97 runs sampled)
esm const binding of named import x 1,294,673,492 ops/sec ±0.35% (97 runs sampled)
cjs const binding of named import x 1,290,601,893 ops/sec ±0.45% (91 runs sampled)
esm const binding of default import x 1,296,853,584 ops/sec ±0.37% (95 runs sampled)
cjs const binding of default import x 1,295,217,256 ops/sec ±0.39% (95 runs sampled)
Fastest is esm namespace import
Done in 71.70s
```