Add support for different VHDL standards

[Schottkyc137]

This adds the initial infrastructure to support different VHDL standards.

Currently, the standards 1993, 2008 and 2019 are implemented.

• Enable specifying the default standard to analyse in the config
  Example:

  standard = "1993" [libraries] ...

• Use different keywords and attributes based on that config

[Schottkyc137]

Per-file and per-library overwrites are planned too.
I'm not entirely sure how to implement this though as the parser is the same for all files at the moment. A per-file parser seems a little inefficient and cumbersome. Maybe a per-file parser with reference to symbol tables is best here, allowing multiple files to share the same symbol table.

[Schottkyc137]

@kraigher there is an AttributeDesignator::Descending attribute defined but I couldn't find it in the standard (regardless of the version). Are you sure this exists?

[kraigher]

If you cannot find it in the LRM it was probably added by mistake.