C++ 20's char8_t isn't supported

[ergrelet]

Hi!

When parsing type declarations containing fields with the primitive type associated with C++ 20's char8_t type, the following error is generated: Type 124 not found.
So the type index appears to be 0x7c, even though it's not been documented in the microsoft-pdb repository.

This comment (and thread) mentions the issue as well: https://github.com/microsoft/microsoft-pdb/issues/51#issuecomment-954876920

That'd be great if support for this primitive type could be added. I can make the PR if needed, as it shouldn't be too much of a change in the code base.