Add formatting, linting, and testing to CI checks (#44)

This PR adds `cargo fmt -- --check`, `nx lint ...` and `nx test ...` as
GitHub Actions. This will cause future PRs to automatically fail when
formatting issues, Clippy warnings, or test failures are present in the
changed code.

.github/workflows/pr-validation.yml

@@ -0,0 +1,55 @@
+name: Run formatter, linter and tests
+on:
+  merge_group:
+  pull_request:
+  push:
+    branches:
+      - main
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  rustfmt:
+    timeout-minutes: 5
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - name: Check Rust formatting
+        run: cargo fmt --all -- --check
+
+  lint-and-test:
+    timeout-minutes: 30
+    strategy:
+      max-parallel: 1
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ runner.os }}-cargo-lint-and-test-${{ hashFiles('**/Cargo.toml') }}
+      - uses: dtolnay/rust-toolchain@stable
+      - name: Install Linux dependencies
+        run: |
+              sudo apt-get update
+              sudo apt-get install -y --no-install-recommends \
+                libasound2-dev \
+                libudev-dev \
+                git
+      - name: Install JS dependencies
+        run: npm ci
+      - name: Ensure unambiguous `main`
+        if: github.ref != 'refs/heads/main'
+        run: |
+              git remote set-branches origin main
+              git fetch --depth 1 origin main
+              git branch main origin/main
+      - name: Run Nx-configured `lint` and `test` targets
+        run: npx nx affected -t lint test --base=main --head=HEAD