Migrate to async LuaMapgen (requires mt 5.9-dev)

[Niklp09]

Minetest 5.9 (luanti-org/luanti@3cac17d) finally introduced async Lua mapgen, this will reduce the lag while exploring significantly.

List of on_generated calls in our current modpack:

• biome_lib/block_que_checks.lua (-> we don't use biome_lib anymore)
• caverealms/init.lua (done in minetest-mirrors/caverealms_lite@029fc23)
• ethereal/decor.lua (done in minetest-mirrors/farming@433d485)
• farming/crops/cocoa.lua (done in minetest-mirrors/farming@433d485)
• moretrees/init.lua (requires access outside of generated block, not possible iirc)
• plantlife; currently only 3 relevant calls for us (maybe possible)

[Niklp09]

Progress is there: https://github.com/Niklp09/minetest-async-it

[Niklp09]

Farming support get reverted due to mem leak issue.
https://codeberg.org/tenplus1/farming/commit/e3bf5f92a7d6faccce0f6e0f8569d43d20ddf897