polish: compact AST output

[JLHwung]

This PR port compact Babel AST output used in Babel parser test fixtures. I am using this PR when working on babel/babel#11807

Here is a preview of error message after this PR, it helps people browsing AST by concatenating loc properties.

Do not reuse nodes. Use `t.cloneNode` (or `t.clone`/`t.cloneDeep` if using babel@6) to copy them.
    {
      "type": "Identifier",
      "name": "N",
      "loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11},"identifierName":"N"}
    }
    Parent:
    [
      {
        "type": "AssignmentExpression",
        "operator": "=",
        "left": {
          "type": "Identifier",
          "name": "N",
          "loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11},"identifierName":"N"}
        },
        "right": {
          "type": "ObjectExpression",
          "properties": []
        },
        "extra": {
          "parenthesized": true,
          "parenStart": 67
        }
      },
      {
        "type": "LogicalExpression",
        "operator": "||",
        "left": {
          "type": "Identifier",
          "name": "N",
          "loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11},"identifierName":"N"}
        },
        "right": {
          "type": "AssignmentExpression",
          "operator": "=",
          "left": {
            "type": "Identifier",
            "name": "N",
            "loc":{"start":{"line":1,"column":10},"end":{"line":1,"column":11},"identifierName":"N"}
          },
          "right": {
            "type": "ObjectExpression",
            "properties": []
          },
          "extra": {
            "parenthesized": true,
            "parenStart": 67
          }
        }
      }
    ]

[Andarist]

@nicolo-ribaudo do you recall why this piece of code is leaving outside of Babel monorepo? I don't mind it, but maybe worth moving it into Babel org or to the monorepo itself? I think I've extracted it to a separate repo as I've wanted to use this check in a couple of other projects (i.e. regenerator), but I see that Babel itself is now depending on this module.

[nicolo-ribaudo]

I think that it's because it's used in regenerator tests. We can move it to the Babel org if you want.

[Andarist]

I'm ok either way - just let me know what works for you best and I will either merge this in or prepare a PR for Babel's monorepo.

[JLHwung]

I and good to move this package to monorepo.

[existentialism]

Agree w/ moving!

[nicolo-ribaudo]

Do we need to move this in the monorepo as a new package, or is it possible to just inline it in the fixtures runner?

[Andarist]

I would be in favor of making this a helper package - it has been quite helpful for me in a few Babel transforms I was working on. I can prepare a PR for this - it would just take me a couple of days before I will be able to do so.