Add support for descriptions on everest.yaml

### I am planning on implementing this myself

No

### Describe your request

Currently the everest.yaml only allows to add the mods name, so it would be great to add support for having a short description of the mod.
That description could be shown to the user on the mod list on everest or from the modded launcher to help better recognize or easily find the purpose of certain mods.
Note that descriptions are already present on the gamebanana pages for mods, but querying the descriptions separate from downloading the mod seems a bit dirty, plus this would only work for publicly available gamebanana mods.

The implementation of this for everest would simply consist on adding a field to the class that holds mod relevant data, and maybe displaying it under the name of the mod on the mod toggle list.

For getting the modder to introduce the description we have two types of mods:
- Code mods: code modders will modify the everest.yaml file manually so the only change would be to add a sample description to the mod templates.
- "Map mods": for modding workflows where the user only uses lönn (or the deprecated ahorn) the description could be asked right after asking for the mod name to the user. (Note: i am not familiar with either of those apps so this is just a guess)

### Additional context

The olympus fna rewrite is being made with support for this feature in mind, and mean while this is just a feature request the descriptions will be fetched from gamebanana when possible.