Versioned mod loaders

**What do you want to see?**
Versioned mod loaders - at the beginning, just a "legacy" (SE4 compatible) mode and a "modern" (new stuff that breaks SE4 mods) mode. Later, multiple versions that may or may not be compatible with each other. These versions need not be numeric or sequential; they could be things like "Bradley" and "kumquat-42", just identifiers for which mod loader to use for a particular mod.

**How would it help players?**
Flexibility in designing mod files without worrying about breaking other mod files. We could have SE4-style text mods, JSON mods, XML mods, C# mods, whatever...

**Alternatives**
???

**Screenshots and mockups**
???

**Things to consider**
???