Butterfly improvements, firefly tweaks, replace firefly/butterfly spawn with LBM

[EmptyStar]

• Improved butterfly textures from RPG16
• Convert butterfly and firefly timer logic to LBM's and move logic to their respective Minetest Game mods from asuna_decor
• Butterflies now also spawn in the Sakura and Grassland biomes
• Set place_offset_y = 2 for fireflies
• Remove default butterfly and firefly biome decorations in favor of asuna_decor decorations