Flowers should use a placeable.json list instead of using hardcoded #dirt

All Minecraft flowers should use a placeable.json list instead of being hardcoded to look for blocks in the #dirt category.

This is already being done with dripleaf in big_dripleaf_placeable/small_dripleaf_pleaceable. Flowers and other greenery/decorators should follow this to allow more simple options for datapack creators.

For example: It's currently impossible to make it so leaves can have flowers placed on top of them without causing adverse side effects such as: podzol being able to replace leaf blocks. Especially troublesome for 2x2 spruce trees where the grass can be low enough to be replaced.