ci: add github actions workflow

Author: bitwalker

.github/ISSUE_TEMPLATE.md

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+### Steps to reproduce
+
+- Configuration Used
+- Errors/Incorrect Behaviour Encountered
+
+### Description of issue
+
+- What are the expected results?
+- Is the documentation incorrect?

.github/PULL_REQUEST_TEMPLATE.md

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+### Summary of changes
+
+I'll review the commits, so I mostly want to understand the "why" rather than the "what"
+
+### Checklist
+
+- [ ] New functions have typespecs, changed functions were updated
+- [ ] Same for documentation, including moduledocs
+- [ ] Tests were added or updated to cover changes
+- [ ] Commits were squashed into a single coherent commit
+- [ ] Notes added to CHANGELOG file which describe changes at a high-level

.github/workflows/elixir.yml

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+name: elixir
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+    env:
+      MIX_ENV: test
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - pair:
+              elixir: "1.17"
+              otp: "27"
+            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: |
+            ${{ runner.os }}-mix-${{matrix.pair.elixir}}-${{matrix.pair.otp}}-
+
+      - run: mix deps.get --only test
+
+      - run: mix format --check-formatted
+        if: ${{ matrix.lint }}
+
+      - run: mix deps.get && mix deps.unlock --check-unused
+        if: ${{ matrix.lint }}
+
+      - run: mix deps.compile
+
+      - run: mix test