Make `weighted`'s TypeScript signature generic

[egilll]

This pull request updates weighted's TypeScript signature to accept other types of arrays.

Weighted just returns a random item from the given array and does not require it to be an array of numbers. I've found this function to be quite useful when selecting from non-numerical arrays.

^{(This change is too minor to be copyrightable, but to alleviate any concerns, I assign all copyright to ChrisCavs)}

[vegeta897]

+1 would like to see this merged!