cmake: add ghostty_vt_add_target() for cross-compilation

Add a ghostty_vt_add_target() CMake function that lets downstream
projects build libghostty-vt for a specific Zig target triple. The
function encapsulates zig discovery, build-type-to-optimize mapping,
the zig build invocation, and output path conventions so consumers
do not need to duplicate any of that logic. It creates named IMPORTED
targets (e.g. ghostty-vt-static-linux-amd64) that work alongside the
existing native ghostty-vt and ghostty-vt-static targets.

The build-type mapping is factored into a shared _GHOSTTY_ZIG_OPT_FLAG
variable used by both the native build and the new function.

The static library targets now propagate c++ as a link dependency on
non-Windows platforms, fixing link failures when consumers use static
linking with the default SIMD-enabled build.

A new example/c-vt-cmake-cross/ demonstrates end-to-end cross-
compilation using zig cc as the C compiler, auto-detecting a cross
target based on the host OS.

.github/workflows/test.yml

@@ -278,6 +278,11 @@ jobs:
       fail-fast: false
       matrix:
         dir: ${{ fromJSON(needs.list-examples.outputs.cmake) }}
+        exclude:
+          # Cross-compilation with zig cc requires a single-config
+          # generator (Makefiles/Ninja). The Windows CI uses Visual
+          # Studio which always uses MSVC and ignores CMAKE_C_COMPILER.
+          - dir: c-vt-cmake-cross
     name: Example ${{ matrix.dir }} (Windows)
     runs-on: namespace-profile-ghostty-windows
     timeout-minutes: 45