feat(bench): change --json output format

[bartlomieju]

Closes #17775

Instead of this:

[
  {
    "runtime": "Deno/1.28.3 x86_64-apple-darwin",
    "cpu": "Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz",
    "origin": "file:///Users/serhiy.barhamon/tmp/deno/test_ffi/tests/bench.js",
    "group": null,
    "name": "Deno.UnsafePointerView#getUint32",
    "baseline": false,
    "result": {
      "ok": {
        "n": 49,
        "min": 1251.9348,
        "max": 1441.2696,
        "avg": 1308.7523755102038,
        "p75": 1324.1055,
        "p99": 1441.2696,
        "p995": 1441.2696,
        "p999": 1441.2696
      }
    }
  },
  {
    "runtime": "Deno/1.28.3 x86_64-apple-darwin",
    "cpu": "Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz",
    // ...
  }
]

We now output this:

{
  "runtime": "Deno/1.28.3 x86_64-apple-darwin",
  "cpu": "Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz",
  "benches": [
    {
      "origin": "file:///Users/serhiy.barhamon/tmp/deno/test_ffi/tests/bench.js",
      "name": "Deno.UnsafePointerView#getUint32",
      "baseline": false,
      "result": {
        "n": 49,
        "min": 1251.9348,
        "max": 1441.2696,
        "avg": 1308.7523755102038,
        "p75": 1324.1055,
        "p99": 1441.2696,
        "p995": 1441.2696,
        "p999": 1441.2696
      }
    },
    {
      // ..
    }
  ]
}

[dsherret]

LGTM. Thanks!