Point users towards correct fix for "missing capability" errors

[jansol]

While trying to get texpresso to compile to SPIR-V I very early on encountered an error about a missing OpCapability Int8.

After some searching around I eventually found out how to fix this, but it would be nice if the error message could point users towards the right API, i.e. in this case SpirvBuilder.capability(spirv_builder::capability::Cap)