Add support to source code assets to reference each other

One asset defined in one DRL should be usable from other DRLs. So we need a way to import them from one file into another, in a similar way as #includes works in C++ or "import" in java, etc.

So, if I declare a type in DRL1.drl:

declare XYZ
...
end

I can then use it in rules in DRL2.drl:

uses "DRL1.drl"

rule "ABC"
when
XYZ(...)
then
end

Same for any other assets like functions.

This feature will require proper dependency management to avoid circular dependencies, classloader issues, etc. Also, the IDE will require update to be able to properly reflect these situations without raising false errors.