Add GitHub Actions workflow to run tests

The new `ci.yml` workflow supposed to run `mix test`, code formatting
check, and code static analysis tool on every push or pull request.

This should simplify our GitHub life.

.github/workflows/ci.yml

@@ -0,0 +1,76 @@
+name: CI
+
+on:
+  - push
+  - pull_request
+
+permissions:
+  contents: read
+
+jobs:
+  mix_test:
+    name: mix test (Elixir ${{matrix.elixir}} / OTP ${{matrix.otp}})
+    runs-on: ubuntu-latest
+
+    env:
+      MIX_ENV: test
+
+    strategy:
+      matrix:
+        include:
+          - elixir: 1.14.5
+            otp: 25.3
+
+          - elixir: 1.16.3
+            otp: 26.2
+
+          - elixir: 1.17.1
+            otp: 27.0
+
+    steps:
+      - name: Setup Elixir
+        uses: erlef/setup-beam@v1
+        with:
+          elixir-version: ${{matrix.elixir}}
+          otp-version: ${{matrix.otp}}
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Cache deps
+        uses: actions/cache@v4
+        env:
+          cache-name: deps-cache
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{hashFiles('**/mix.lock') }}
+          restore-keys: ${{ runner.os }}-mix-${{ env.cache-name }}-
+
+      - name: Cache build
+        uses: actions/cache@v4
+        env:
+          cache-name: compiled-build-cache
+        with:
+          path: _build
+          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{hashFiles('**/mix.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-mix-${{ env.cache-name }}-
+            ${{ runner.os }}-mix-
+
+      - name: Install deps
+        run: mix deps.get
+
+      - name: Compile with warnings as errors
+        run: mix compile --warnings-as-errors
+
+      - name: Check code formatting
+        run: mix format --check-formatted
+
+      - name: Check for unused dependencies
+        run: mix deps.unlock --check-unused
+
+      - name: Analyze code
+        run: mix credo suggest --ignore todo
+
+      - name: Run tests
+        run: mix test