Run a function every time the library loads, hot reloads included, once
its classes are registered: the place to register console commands and
variables, which an unload removes.
Attribute macro for defining a Rust struct as a UE struct: a data table’s
row, a property’s value. Every field is a #[uproperty]; the struct’s
memory is UE’s, read and written through the generated <Name>Ext trait
on UStructRef<Name> and OwnedStruct<Name>.