VHDL-2019 support

[GCHQDeveloper888]

There is currently no VHDL-2019 support for the dev tools I use:

• Riviera pro
• Vivado
• Quartus
• Modelsim

I've raised a PR to add this functionality: #466
I can confirm that the 4 tools above will now handle -2019 files, but some of them do now support all of the -2019 features.

Others will have to add functionality for other tools that I'm not familiar with/don't have licenses for.