feature: support for json5 style comments

[glowinthedark]

jless throws an error with JSON5-style comments:

[
  {
    "operation": "shift",
    "spec": {
      "rating": {
        "primary": {
          // simple match.  Put the value '4' in the output under the "Rating" field
          "value": "Rating",
          "max": "RatingRange"
        },
        // match any children of "rating"
        // Shiftr has a precendence order when matching, so the "*" will match "last".
        // In this case anything that isn't "primary".
        "*": {
          // &1 means, go up one level and grab that value and substitute it in
          //  in this example &1 = "quality"
          "max":   "SecondaryRatings.&1.Range",
          "value": "SecondaryRatings.&1.Value",
          //
          // We want "quality" to be a value field in the output under
          //  "SecondaryRatings.quality.Id", but "quality" is an input key not an input value.
          // The "$" operator means use the input key, instead of the input value as ouput
          "$": "SecondaryRatings.&1.Id"
        }
      }
    }
  }
]

Comments are, of course, not a part of the standard JSON spec, but it would be great to be able to have some support for certain JSON5 extensions such as comments.

[PaulJuliusMartinez]

I expect to add support for this sometime in the next couple of months.

[glowinthedark]

@PaulJuliusMartinez: given that JSON5 is apparently a more expensive grammar and, for example, python's json5 3rd party module (https://pypi.org/project/json5/) is significantly less performant than 'native' plain JSON ("1000-6000x slower than the C-optimized JSON module" according to the project page), so it might also be slower with rust JSON5 implementations; maybe it would make sense to have the ability to switch parser engines, and enable JSON5 support with a flag at the expense of performance overhead?

[PaulJuliusMartinez]

Yes, this is one of the primary concerns I am considering as I work on implementing this.