Improve deno bench --json output before 1.31 release

[dsherret]

Instead of an array like so:

[
  {
    "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",
    // ...
  }
]

It would probably be better to not duplicate the runtime and cpu for each object and instead do something like:

{
  "version": 1, // https://github.com/denoland/deno/issues/17774
  "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
      }
    },
    {
      // ..
    }
  ]
}

We should think of some other improvements as well, like no "ok" sub object in "result" and not serializing the group name when it's "null" or perhaps grouping together benches with the same group?

Related #17774