Standardize filename of external libraries

[t-sommer]

Currently the filename of the shared or static library that contains the implementation of an external function is implicitly defined by the example directory structure in 12.9.4 Annotations for External Functions:

I propose to add an explicit rule how to derive the filename of the library from the Library element in the annotation or at least add darwin/macOS to the example.

Note that, unlike FMI, Modelica requires a lib prefix on Linux. Maybe this could also be harmonized.