ci: setup basic continuous integration

This currently just checks formatting and runs the test-suite, but we
have enough tests now that is useful to start.

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: nixbuild/nix-quick-install-action@v28
+      - name: Setup Cache
+        uses: DeterminateSystems/magic-nix-cache-action@v8
+      - name: Enter Devshell
+        run: . <(nix print-dev-env "$(nix-instantiate shell.nix)"
+      - name: Check Format
+        run: treefmt --ci
+      - name: Run Tests
+        run: cargo test --all