Flora spread: Allow spread on rainforest litter. Other improvements

Flora spread: Allow spread on rainforest litter. Other improvements

Use the soil group more instead of checking for multiple node names.
Remove 'neighbors' from ABM.
Turn any flora to dry shrub if on a non-soil, except when on default:sand
to avoid dune grasses being replaced.
Search for "group:soil" when searching for a position for the new flora
node, instead of searching for multiple node names, however do not spread
flora onto desert sand, which is in the soil group.

Remove default:dirt_with_snow from the soil group as it would be frozen
soil. It can be dug and placed to turn it into dirt (consider this some
extra work needed to make it cultivatable).