Fireflies API: An idea to resolve issue #2966

[JoseDouglas26]

This is my initial idea to resolve the issues reported in issue #2966. Using the fireflies API, mods like Ethereal and Extra Biomes can register the decoration of fireflies in their custom biomes.

[JoseDouglas26]

The idea is that this API evolves to allow greater control over recording firefly decorations.

[appgurueu]

The basic idea of this PR - adding an API to register firefly decorations - is good. However, please remove redundant comments, please document the API in game_api.txt.

Code-wise, three things seem fishy to me: (1) Reusing decoration names; (2) not using the biomes parameter when registering decorations for mgv6; (3) the two calls to register_decoration seem fishy; I believe there should ultimately only be one if this is refactored properly.

Design-wise, all these functions basically do is set some different defaults for deco registrations and do two registrations, one for high, one for low. There's some potential for deduplication here. What if you made the function take a table of "overrides", which would be applied "on top" of the default values?

Ultimately, I frankly also have to say that I think this is low priority. Without this, mods that register biomes just have to register the decorations themselves (which also gives them finer-grained control).