feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

.github/workflows/test.yml

@@ -201,6 +201,20 @@ jobs:
         name: Show logs
         run: cat $(find "$LOG_DIR" -type f)
 
+  zig-build:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 45
+    name: build using zig build
+    steps:
+      - uses: actions/checkout@v4
+      - uses: mlugg/setup-zig@v1
+        with:
+         version: 0.14.0
+      - run: sudo apt-get install -y inotify-tools
+      - run: zig build test_nlua0
+      - run: zig build nvim_bin && ./zig-out/bin/nvim --version
+      - run: zig build functionaltest
+
   windows:
     uses: ./.github/workflows/test_windows.yml