chore(tools): add CI through GHA

.github/workflows/elixir.yml

@@ -0,0 +1,85 @@
+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+
+name: Elixir CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+
+env:
+  MIX_ENV: test
+
+jobs:
+  test:
+    name: Test (Elixir ${{ matrix.elixir }}, OTP ${{ matrix.otp }})
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        # https://hexdocs.pm/elixir/compatibility-and-deprecations.html#compatibility-between-elixir-and-erlang-otp
+        include:
+          # Newest supported Elixir/Erlang pair.
+          - elixir: '1.17'
+            otp: '27.1'
+            lint: true
+            dialyzer: true
+
+          # One version before the last supported one.
+          - elixir: '1.16'
+            otp: '26.2'
+
+          - elixir: '1.15'
+            otp: '22.3.4'
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Elixir and Erlang
+      uses: erlef/setup-beam@61e01a43a562a89bfc54c7f9a378ff67b03e4a21 # v1.16.0
+      with:
+        elixir-version: ${{ matrix.elixir }}
+        otp-version: ${{ matrix.otp }}
+
+    # Manually restore and then save, so that we can save the "_build" directory
+    # *without* the Elixir compiled code in it.
+    - name: Restore Mix dependencies cache
+      uses: actions/cache/restore@v4
+      id: mix-deps-cache
+      with:
+        path: |
+          _build
+          deps
+        key: ${{ runner.os }}-${{ matrix.otp}}-${{ matrix.elixir }}-mix-${{ hashFiles('**/mix.lock') }}
+        restore-keys: ${{ runner.os }}-${{ matrix.otp}}-${{ matrix.elixir }}-mix-
+
+    - name: Install and compile Mix dependencies
+      if: steps.mix-deps-cache.outputs.cache-hit != 'true'
+      run: mix do deps.get --check-locked, deps.compile
+
+    - name: Save Mix dependencies cache
+      uses: actions/cache/save@v4
+      if: steps.mix-deps-cache.outputs.cache-hit != 'true'
+      with:
+        path: |
+          _build
+          deps
+        key: ${{ steps.mix-deps-cache.outputs.cache-primary-key }}
+
+    - name: Check formatting
+      if: matrix.lint
+      run: mix format --check-formatted
+
+    - name: Check compiler warnings
+      if: matrix.lint
+      run: mix compile --warnings-as-errors
+
+    - name: Run tests
+      run: mix test