Support include directory for implicitly available files

[MarkusAmshove]

Natural has multiple LDAs, PDAs etc. that are implicitly available to the runtime but not in the repository.
NaturalONE handles this with its connection to the NDV server, so that it can see the definition from the system libraries.

To support all checks that natparse does for calls to these modules, we could add support for a include/ directory in the repository.

This folder resides side by side to Natural-Libraries and can contain libraries like SYSTEM, SYSRPC etc.

In there, a user can put stubs for data areas and subprograms (define data only) to make them visible to the parser.

include/SYSRPC
include/SYSTEM
Natural-Libraries/LIB1
Natural-Libraries/LIB2

[Claes65]

I solved this by putting a folder inside the top STEPLIB (often SYSTEM) containing the missing files as stubs.