Add leftRightIntoCmdGoes: 'up'/'down' option

By default, the Left and Right keys move the cursor through all possible cursor
positions in a particular order: right into a fraction puts the cursor at the
left end of the numerator, right out of the numerator puts the cursor at the
left end of the denominator, right out of the denominator puts the cursor to the
right of the fraction; symmetrically, left into a fraction puts the cursor at
the right end of the denominator, etc. Note that right out of the numerator to
the left end of the denominator is actually leftwards (and downwards, it's
basically wrapped). If instead you want right to always go right, and left to
always go left, you can set `leftRightIntoCmdGoes` to `'up'` or `'down'` so that
left and right go up or down (respectively) into commands, e.g. `'up'` means
that left into a fraction goes up into the numerator, skipping the denominator;
symmetrically, right out of the numerator skips the denominator and puts the
cursor to the right of the fraction, which unlike the default behavior is
actually rightwards (the drawback is the denominator is always skipped, you
can't get to it with just Left and Right, you have to press Down); which is
the same behavior as the Desmos calculator. `'down'` instead means it is the
numerator that is always skipped, which is the same behavior as the Mac OS X
built-in app Grapher.