chore: add llvm-cov to CI

.github/workflows/coverage.yml

@@ -0,0 +1,29 @@
+name: Coverage
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Install Rust toolchain (nightly)
+        run: rustup toolchain install nightly --component llvm-tools-preview
+
+      - name: Install cargo-llvm-cov
+        run: cargo install cargo-llvm-cov
+
+      - name: Run coverage
+        run: cargo llvm-cov --report-html --lcov --output-path=coverage
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v3
+        with:
+          name: coverage-report
+          path: coverage/