Add support for ignoring/blackboxing libraries or DU names

[Bonusbartus]

When running projects with pre-compiled/vendor libraries or mixed language projects, not all files will be in vhdl libraries added to the vhdl_ls.toml file. It would be very useful to be able to add primary units with "missing declarations" and unknown libraries, to an ignore list;

for unknown libraries (lib_x) it would be an option to add them to the vhdl_ls.toml file without any files, in which case all: "use lib_x.pkg" statements would be skipped by the semantics checker

[kraigher]

This would be quite easy. It is a good first issue for someone who wants to. contribute to the project.

[patgro1]

Same as #118 ??