zig 0.12 migrations

[markjoshwel]

1. std.os has been renamed to std.posix
   as such, as per the release notes nuko will henceforth use std.process.exit

2. lowercase enum fields

   Zig 0.12.0 takes the opportunity to adjust the field names of some enums in std.builtin to align with our current naming conventions, which dictate that enum fields should be snake_case. The following enums have been updated: