Swap out the moon emoji used for progress spinner for a single-cell character.

The moon emoji looked cool, but couldn't be used because of bugs in xterm.js, used for VSCode's terminal, among others. The moon emoji is two character cells wide, but xterm.js doesn't advance by two cells when it adds the emoji, but does go back by two when it backspaces.

This changes us to a different character animation (dots) that is only one cell wide, and so doesn't have this problem.