Introduce CI workflow with cross-platform support

- Introduce a new CI workflow to enhance automatic testing on multiple platforms.
- Ensure CI triggers for all pull requests and push events to the main branch.
- Implement Nix for uniform build and test environment across different systems.

.github/workflows/ci.yaml

@@ -0,0 +1,33 @@
+name: build test
+on:
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+concurrency:
+  # Cancels pending runs when a PR gets updated.
+  group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}
+  cancel-in-progress: true
+jobs:
+  build-test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [macos-latest, ubuntu-latest]
+    runs-on: ${{matrix.os}}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@v30
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      # Run our checks to catch quick issues
+      - run: nix flake check
+
+      - name: build & test
+        run: nix develop -c zig build test --summary all