Drop istanbul, add c8

Author: perry-mitchell

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
                   node-version: ${{ matrix.node-version }}
             - run: npm ci
             - run: npm run build
-            - run: npm run test
+            - run: npm run test:coverage
     types:
         runs-on: ubuntu-latest
         strategy: