ci: add basic CI workflow

Runs cargo check, build, and test on ubuntu and macOS using the nightly
toolchain (required by parser's use of if_let_guard).

.github/workflows/ci.yml

@@ -0,0 +1,53 @@
+on: [push, pull_request]
+
+name: Basic CI
+
+env:
+  CARGO_TERM_COLOR: always
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
+jobs:
+  check:
+    name: cargo check
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+    steps:
+      - uses: actions/checkout@v6
+      - uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo check --workspace
+
+  build:
+    name: cargo build
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+    steps:
+      - uses: actions/checkout@v6
+      - uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo build --workspace
+
+  test:
+    name: cargo test
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+    steps:
+      - uses: actions/checkout@v6
+      - uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo test --workspace