Exponent properties 1

Here's an exercise for the not-yet-exercised video "Exponent properties 1".

Important note: this includes a change to expressions.js that is likely to break things elsewhere. (See this pull request: https://github.com/Khan/khan-exercises/pull/44625)

I've also added a function to expressions.js that expands integer power expressions. I'm new to the code, so if you can recommend a better place for this, I'm all ears!

Also, it strikes me that the auto-paren insertion is a little aggressive. Things become pretty paren-y pretty fast. Is there a better way of dealing with this in the code already? If not, would you guys be interested in a patch that changed it so that parens were inserted manually and not automatically? Something like <code>expr(["*", 2, ["paren", ["+", "A", "B"]]])</code> yielding <code>2(A+B)</code> ?

If so, I'd be happy to implement it, but I imagine it would alter all of the existing exercises, which seems like a lot of work to fix!

If not, is there a standard means of using some kind of latex-like language inside your framework? Apologies if I missed such information in the docs.

Anyway, sweet framework! I look forward to using it more! :)