Add Property for recursive mod loading

[notTamion]

fixes #81
This pr adds the -Dfabric.loadModsRecursively system property which will cause the default mods folder to be loaded recursively. Paths containing "disabled" are ignored. This way we don't break existing setups which use a subfolder with any name to make sure mods are not loaded but provide users with an option to easily categorize their mods

[modmuss50]

This can already be done with -Dfabric.addMods=mods

See: #470

[notTamion]

Yes, however the addMods property does not allow you to easily disable entire folders without having to rename all the files or moving them outside the mod folder completetly, also having the mods folder scanned twice feels rather hacky to me.
If you don't want to add another property for this we could still check the paths for "disabled" in ArgumentModCandidateFinder

[sfPlayer1]

Maybe I just just add a path filter regex or so -_-