feat: creation of a new onCircular hook for accumulating circular refs

[erunion]

This creates a new options.dereference.onCircular hook that will be executed when a circular $ref is detected, and similar to onDereference, will allow consumers to accumulate a list of circular references within their schemas.

I have written a test for this, and also updated a few test case typos where they were using the legacy options.$refs property instead of options.dereference.

Closes out #358

[coveralls]

Pull Request Test Coverage Report for Build 12918473069

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 92.04%

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Totals
Change from base Build 12190033497:	-0.03%
Covered Lines:	1569
Relevant Lines:	1676

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💛 - Coveralls

[jonluca]

Looks great! Thanks for the PR