Add first GitHub CI setup (#28)

* Implement CI

* Try to run the workflow from my branch

Unsure about the syntax at this point.

* Revert "Try to run the workflow from my branch"

This reverts commit ef7b26f.

* Test running on branch itself?

* Trying more things

---------

Co-authored-by: Thomas Millar <[email protected]>

.github/workflows/ci.yml

@@ -0,0 +1,59 @@
+name: CI
+
+on:
+  workflow_dispatch:
+  pull_request:
+    types:
+      - *
+  push:
+    branches:
+      - main
+      - setup-ci
+
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+    env:
+      MIX_ENV: test
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - pair:
+              elixir: "1.11"
+              otp: "21.3"
+          - pair:
+              elixir: "1.16"
+              otp: "26.2"
+            lint: lint
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: erlef/setup-beam@v1
+        with:
+          otp-version: ${{matrix.pair.otp}}
+          elixir-version: ${{matrix.pair.elixir}}
+
+      - uses: actions/cache@v4
+        with:
+          path: |
+            deps
+            _build
+          key: ${{ runner.os }}-mix-${{matrix.pair.elixir}}-${{matrix.pair.otp}}-${{ hashFiles('**/mix.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-mix-
+
+      - run: mix deps.get --check-locked
+
+      - run: mix format --check-formatted
+        if: ${{ matrix.lint }}
+
+      - run: mix deps.unlock --check-unused
+        if: ${{ matrix.lint }}
+
+      - run: mix deps.compile
+
+      - run: mix compile --warnings-as-errors
+        if: ${{ matrix.lint }}
+
+      - run: mix test