* Fix compilation on Java versions other than 8
* Add basic support for NeoForge
This uses Forge's built-in fml.forgeGroup flag to distinguish Forge and NeoForge's package names. Consequently, this flag's value needs to be passed to most installer-related functions.
* Update Javadocs with Forge group parameter
* Add Forge group support for manual installs
* Assume we're using NeoForge if `--fml.forgeGroup` isn't specified
I was hoping not to hardcode any package names here, but this is the only way to unbreak manual installs without unintuitively asking users to add the flag themselves.
