Add natvis for better debug view

[duckdoom5]

I'd like to see a natvis added, to improve the display of the values in the (msvc) debugger.

Taking inspiration from the msvc STL natvis I think it should be possible to display the values as the stl unordered map would. See https://github.com/microsoft/STL/blob/8dc4faadafb52e3e0a627e046b41258032d9bc6a/stl/debugger/STL.natvis#L1380

[viable-alternative]

Created PR#135 with natvis file and a demo VS2022 project.

[duckdoom5]

Awesome work. Thank you for sharing!