Update to Zig 0.16.0

This commit represents the majority of the work necessary to upgrade
Ghostty to use Zig 0.16.0.

Key parts:

* In addition to its previous responsibilities, the global state now
  houses state for global I/O implementations and the process
  environment. It is now also utilized in the main application along
  with the C library. Where necessary, global state is isolated from key
  parts of the implementation (e.g., in libghostty subsystems), and it's
  expected that this list will grow.

* We currently manage our own C translation layer where necessary. In
  these cases, cImport has been removed in favor of the new external
  translate-c package. Due to fixes that have needed be made to properly
  translate the dependencies that were swapped out, as mentioned, we
  have had to backport fixes from the current translate-c package (and
  the upstream Arocc dependency). We will host this ourselves until Zig
  0.17.0 is released with these fixes.

* Where necessary (only a small number of cases), some stdlib code from
  0.15.2 (and even from 0.17.0) has been taken, adopted, and vendored in
  lib/compat.

Co-authored-by: Leah Amelia Chen <hi@pluie.me>

src/main.zig

@@ -22,5 +22,10 @@ else
     .{};
 
 test {
+    // Zig 0.16.0 has made test logging more strict. Now, *anything* that gets
+    // printed to stderr results in a "failed command" message, even if the
+    // tests ultimately passed. To reduce confusion here (and honestly, test
+    // log spam in general), we bump the default testing log level to error.
+    std.testing.log_level = std.log.Level.err;
     _ = entrypoint;
 }