Update Zig toolchain

[mcadariu]

PR to close #104

[urso]

Hi, thank you for giving this a try.

The removal of usingnamespace is quite unfortunate and we essentially end up with pg.c.c here. As @cImport will also go away I was thinking to move the header translation to build.zig and finally expose that module via pg.c.

The project also uses nix to provide a stable development environment. See flake.nix. We need to update zig-stable to 0.15.2 and use nix flake update to update zig-overlay and zls dependencies. Otherwise this change will break downstream projects.

[mcadariu]

@urso thanks for your feedback, I will try it out.