Build the default CMake target now that helptag generation is
working again.
For build artifacts create a zip file with an instalation of
Neovim (generated by cpack).
- Build for MinGW x86/x86_64. Move build scripts out of the yml file into
separate batch files.
- The MinGW builds use MSYS to get runtime dependencies, but they do not
link against the POSIX adaptation layer.
- For now only build the nvim.exe binary, but not the helptags.
