stop infinite recursion w/ crawling external refs

[cody-greene]

This will fix a few cases of "Uncaught RangeError: Maximum call stack size exceeded"

Specifically if you have a schema with a circular reference like this:

const Node = {
  type: 'object',
  properties: {
    child: {$ref: '#'}
  }
}

And for whatever reason this gets dereferenced twice:

// First call runs fine.
const dereferenced = await $RefParser.dereference(Node)

console.log(dereferenced)
//{
//  type: 'object',
//  properties: { child: [Circular *1] }
//}

// Infinite recursion will occur in resolve-external.js:crawl()
await $RefParser.dereference(dereferenced)

And of course this can happen without calling dereference() twice. In fact, I can construct a dereferenced schema myself like so:

const Node = {
  type: 'object',
  properties: {
    child: null  
  }
}
Node.properties.child = Node

// Infinite recursion will occur 
await $RefParser.resolve(Node)

To resolve this we can modify the recursive function to track seen objects and return early if one is seen second time (or more).

[P0lip]

Looks good!

[github-actions]

🎉 This PR is included in version 9.0.8 🎉

The release is available on:

• v9.0.8
• GitHub release

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