feat: add GitHub workflow for testing

Tests against the last three versions of OTP and Elixir

.github/workflows/elixir.yml

@@ -0,0 +1,37 @@
+name: Elixir CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+
+    name: Build and test Elixir ${{matrix.elixir}} / OTP ${{matrix.otp}}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        otp: ['24.3', '25.3', '26.0']
+        elixir: ['1.13', '1.14', '1.15']
+    steps:
+    - uses: actions/checkout@v3
+    - name: Set up Elixir
+      uses: erlef/setup-beam@v1
+      with:
+        elixir-version: ${{matrix.elixir}}
+        otp-version: ${{matrix.otp}}
+    - name: Restore dependencies cache
+      uses: actions/cache@v3
+      with:
+        path: deps
+        key: ${{ runner.os }}-E${{matrix.elixir}}-O${{matrix.otp}}-mix-${{ hashFiles('**/mix.lock') }}
+        restore-keys: ${{ runner.os }}-E${{matrix.elixir}}-O${{matrix.otp}}-mix-
+    - name: Install dependencies
+      run: mix deps.get
+    - name: Run tests
+      run: mix test