Moss variants should no longer be flammable.

This seems like a minor nitpick, but it would be good for the game for three reasons.

1) Forest fires in pale gardens will now only burn trees instead of leaving craters in the ground.

2) This allows people to use moss instead of grass in their own terraforming projects without a risk of the entire thing going up in flames because of one misclick.

3) Living moss is too wet to burn in real life, so it makes more sense intuitively for players.