prototype exercise on squares and square roots (v.2)

This is a revised “version 0.2” of the exercise I posted on July 1 at https://github.com/Khan/khan-exercises/pull/25255  This version incorporates the first round of comments by Christi Rockwell and Emily Eisenberg. I retroactively created a branch for this, hope it is not a problem that the original source file was in my master branch. (Please bear with me, I'm still learning git, javascript and khan-exercises.)

As I said in the previous version: “The exercise basically asks the student to square, or take the square root of, a given number. It provides a interactive tool to do the actual computation. The style is a bit like the Unit Circle exercise: a numeric answer is read from an interactive diagram, requiring a little interpretation of what is going on.”

Changes since “version 0.1”:
* use LaTex code instead of superscript 2 glyph (Christi, does this fix your Firefox problem?)
* changed TILDA -> TILDE
* removed the “liberal spacing” (not sure I fully understand the conventions yet)
* changed use of toFixed (still do it at top to avoid repeating the display reformatting code)
* moved “You can adjust the diagram...” from hints to the problem statement.
* made initial square size = 3 (labels appear after movement, otherwise initial diagram can appear to misstate the problem)
* made label text (“area=”) not be italic
* changed function definition syntax (requiring function to be defined lexically before use)
* removed optional plus signs in +/- coordinates
* changed unicode ~ to LaTex \\sim in hint to indicate inexact value due to roundoff