Support linking with DPI C/C++ files for simulation flows

[Toucanfan]

This is necessary for some vendor primitives to work correctly under some simulation scenarios (typically IPs), e.g. in Quartus:

$QUARTUS_INSTALL_DIR/eda/sim_lib/simsf_dpi.cpp

DPI files possibly need their own file type in the project model.
Simulation flows need updating to support linking with DPI files, e.g. with -sv_lib in Questasim.

A suitable mechanism should be added on the parser side as well for declaration of DPI files.