add optional isDeepCopy to shuffle & 2 tests for the same

[sanaagarwal]

To address the issue in #2808 and its comments, I added the optional isDeepCopy input parameter to the shuffle method. Now if people want to have a deep copy of the array that's to be shuffled, they can simply set it to be true, while the default only returns a shallow copy or they can manually set it to false.

I added 2 tests in the the randomization test suite for the same.

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[bjoluc]

Thanks @sanaagarwal and sorry for the wait. I'd prefer simply exposing the internal deepCopy utility function, so people can deep-copy whatever they need to. @jodeleeuw WDYT?

[jodeleeuw]

I agree. I think it is semantically cleaner to have a distinct method. Otherwise it's not clear which methods should have this optional parameter and which shouldn't (e.g., sampleWithoutReplacement?)

[bjoluc]

Working on a PR to expose deepCopy now – hang on @sanaagarwal 🙃

[bjoluc]

@sanaagarwal I just noticed deepCopy is already available as jsPsych.utils.deepCopy(), so feel free to use that and we'll update the docs.