Move "Hints" section in all exercises to a separate file

[nik-rev]

I feel like it's too easy to accidentally peek at the hints when they are simply right there. Moving them to a new file would mean that people will have to make the conscious effort to look at the hints. I just think this makes more sense than placing them right in the README file

[thatblindgeye]

Rather than moving hints to a separate file, we should probably take stock as to whether hints are necessary for some of these files. Ideally the lesson(s) leading up to when the exercises are assigned would provide the necessary information for users, though there may be some instances where a hint is necessary to be readily viewable (e.g. if we explicitly want to tell users to use a loop if it's not clear that that is the intent).

[MaoShizhong]

Agreed with what @thatblindgeye said.
Related is #460 where I feel instead of providing String.prototype.repeat as an alternative solution, the "hint" about loops should be moved to the instructions, where a mention of the repeat method could be placed (and instructed to use loops, not that method).

Same might apply for things in other exercises. But I'd rather keep the hints in the READMEs and just evaluate what hints we have.