feat: Add cross platform CI (#6)

--- BEGIN PROMPT ---
Task:
Add a GitHub CI config

Requirements:
The test matrix should include testing on Linux, macOS and Windows.
For Linux, there should be a CI run on the latest Ubuntu version without using Nix and another run based on the Nix setup present in the repository and running on NixOS.
--- END PROMPT ----

Remarks: Since GitHub doesn't offer NixOS runners at the moment, we carry out the Nix tests on Ubuntu.
Future directions: We might migrate to a self-hosted NixOS runner.

Author: zah

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  tests:
+    name: Tests on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install make on Windows
+        if: runner.os == 'Windows'
+        run: choco install make -y
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.x'
+      - name: Run tests
+        shell: bash
+        run: make test
+
+  nix-tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-24.05
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+      - name: Run tests via Nix
+        run: nix develop --command make test