Add Criterion benchmarks for parser (#7686)

This PR sets up [Criterion](https://github.com/bheisler/criterion.rs)
for benchmarking in the main `nu` crate, and adds some simple parser
benchmarks.

To run the benchmarks, just do `cargo bench` or `cargo bench -- <regex
matching benchmark names>` in the repo root:

```bash
〉cargo bench -- parse
...
     Running benches/parser_benchmark.rs (target/release/deps/parser_benchmark-75d224bac82d5b0b)
parse_default_env_file  time:   [221.17 µs 222.34 µs 223.61 µs]
Found 8 outliers among 100 measurements (8.00%)
  5 (5.00%) high mild
  3 (3.00%) high severe

parse_default_config_file
                        time:   [1.4935 ms 1.4993 ms 1.5059 ms]
Found 11 outliers among 100 measurements (11.00%)
  7 (7.00%) high mild
  4 (4.00%) high severe
```

Existing benchmarks from `nu-plugin` have been moved into the main `nu`
crate to keep all our benchmarks in one place.

crates/nu-plugin/README.md

@@ -1,6 +0,0 @@
-# nu-plugin
-
-## Benchmark
-Here is a simple benchmark for different protocol for encoding/decoding nushell table, with different rows and columns.  You can simply run `cargo bench` to run benchmark.
-
-The relative html report is in `target/criterion/report/index.html`.