define used vim default colors for easy reusage on semantic token highlighting

[matu3ba]

Semantic token highlighting groups were recently merged in neovim and overwrite the vim defaults, which makes usage of different colors to signal things or potential fixups etc more cumbersome.

Task: Define the used colors in

zig.vim/syntax/zig.vim

Line 247 in c33e1e3

highlight default link zigDecNumber zigNumber

and provide a simple table in the README, so users have a simpler job to tailor or fix their/others color schema.