Property `rtlfiles` to replace `vlogmodulefiles` and `vhdlentityfiles`

[Roenski]

Introduce a new rtl propety, rtlfiles, to hold both verilog and vhdl files.

Compile order is determined by the order of modules in rtlfiles. Thus, you can now define compile order, with an arbitrary mix of vcom and vlog commands.

This MR also contains a quality-of-life improvement, which will print out the error message produced by simulator, instead of the command that caused it.

[Roenski]

Models sv, icarus work, vhdl works with lang=vhdl, but not with lang=sv, however that bug is not caused by this commit.

[Roenski]

ghdl works in CI