A quick follow-up to https://github.com/nushell/nushell/pull/7686. This
adds benchmarks for evaluating `default_env.nu` and `default_config.nu`,
because evaluating config takes up the lion's share of Nushell's startup
time. The benchmarks will help us speed up Nu's startup and test
execution.
```
eval default_env.nu time: [4.2417 ms 4.2596 ms 4.2780 ms]
...
eval default_config.nu time: [1.9362 ms 1.9439 ms 1.9523 ms]
```
