Zig 0.15 migration

[pluiedev]

Zig 0.15 has been released — though due to its massive breaking changes regarding the standard library (defaulting to unmanaged lists, Writergate, etc.) we're staying with 0.14.1 for the 1.2 release (and any patch updates thereof).

PR over at #8372

As usual, we need to update our dependencies:

• libxev - Updated to 0.15 with fix: compatibility with 0.15 mitchellh/libxev#180
• libvaxis - Main branch has been updated to Zig 0.15 to a state where our use cases work
• z2d - Has a 0.15 branch: https://github.com/vancluever/z2d/tree/zig-0.15.0
• zig-objc - Update to Zig 0.15 mitchellh/zig-objc#26
• zig-js - update to Zig 0.15 mitchellh/zig-js#7
• ziglyph - Patched branch over at https://codeberg.org/pluiedev/ziglyph/src/branch/zig-0.15
• zig-wayland - Updated to 0.15 as of 0.5.0-dev
• zf - Update to Zig 0.15 natecraddock/zf#81
• zig-gobject - feat: update to Zig 0.15 ianprime0509/zig-gobject#119

Other issues (non-blocking)

• Update zig2nix to 0.15 (nix: update zon2nix #8370)

[pluiedev]

#8372 is now ready for review!

[mitchellh]

Ghostty main is now on Zig 0.15