Add a coverage job to CI

This new job is very barebones at the moment. It only outputs a summary
of the coverage in the terminal, since the entire coverage is a bit long
and unwieldy. Perhaps Codecov or similar can be used in the future.

.github/workflows/test.yml

@@ -127,3 +127,20 @@ jobs:
         cd lib/loader
         npm run asbuild
         npm run test
+  coverage:
+    name: "Coverage"
+    runs-on: ubuntu-latest
+    needs: check
+    steps:
+    - uses: actions/checkout@v3
+    - uses: dcodeIO/setup-node-nvm@master
+      with:
+        node-version: current
+    - name: Install dependencies
+      run: npm ci --no-audit
+    - name: Build
+      run: npm run build
+    - name: Collect coverage
+      run: npx c8 -r none -- npm test
+    - name: Output coverage summary
+      run: npx c8 report -r text-summary