Create GitHub Actions worflow for Elixir

Auto-generated code by writing choosing a "recommended Elixir
workflow".

.github/workflows/elixir.yml

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+# 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: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Build and test
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Elixir
+      uses: erlef/setup-beam@61e01a43a562a89bfc54c7f9a378ff67b03e4a21 # v1.16.0
+      with:
+        elixir-version: '1.15.2' # [Required] Define the Elixir version
+        otp-version: '26.0'      # [Required] Define the Erlang/OTP version
+    - name: Restore dependencies cache
+      uses: actions/cache@v3
+      with:
+        path: deps
+        key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
+        restore-keys: ${{ runner.os }}-mix-
+    - name: Install dependencies
+      run: mix deps.get
+    - name: Run tests
+      run: mix test