chore: add Codecov workflow

Author: isometry

.github/workflows/codecov.yml

@@ -0,0 +1,18 @@
+name: Codecov
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    # The branches below must be a subset of the branches above
+    branches: [ "main" ]
+
+jobs:
+  coverage:
+    name: Codecov
+    runs-on: ubuntu-latest
+    steps:
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v3
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}