Add isSubsetByComparator JavaScript array function

[GrayedFox]

This pull request adds a JavaScript array method called isSubsetByComparator which evaluates if an array is a subset of another array using a comparator function.

Happy hacktoberfest and keep up the good work 👍🏾

[Chalarangelo]

Thanks for contributing, but this is a duplicate of isContainedIn.

[GrayedFox]

Hmm, it is true that this method achieves the same result but only when the comparator function looks for an exact match.

This method has added functionality in that when calling the method the user gets to define the match (as opposed to it being a direct match). This function can therefore be used to define the matching behaviour, which is not possible with the isContainedIn() function. See the second example:

isSubsetByComparator((x, y) => x * 2 === y, [1, 3, 5], [2, 4, 6, 8, 10]); // 'true'

@Chalarangelo would you mind taking another look at this PR? I understand it's a busy time for all of you maintainers, considering it's Hacktoberfest, so I don't mean to be pushy or anything.

I might be mistaken in believing that this is not a true duplicate of isContainedIn. Cheers! 🍺

[GrayedFox]

@Trinityyi any chance of getting a 2nd pair of eyes on this? Thank you! 🙇🏾‍♀️ 👓