Add test coverage (#1818)

* test coverage with c8

* use npx for c8

---------

Co-authored-by: Mike Bostock <[email protected]>

Author: Fil

.gitignore

@@ -1,4 +1,5 @@
 .DS_Store
+coverage/
 dist/
 docs/.vitepress/cache
 docs/.vitepress/dist

package.json

@@ -28,6 +28,7 @@
   ],
   "scripts": {
     "test": "yarn test:mocha && yarn test:tsc && yarn test:lint && yarn test:prettier",
+    "test:coverage": "npx c8 yarn test:mocha",
     "test:mocha": "mkdir -p test/output && mocha 'test/**/*-test.*' 'test/plot.js'",
     "test:lint": "eslint src test",
     "test:prettier": "prettier --check src test",
@@ -74,6 +75,11 @@
     "vite": "4.4.7",
     "vitepress": "1.0.0-beta.7"
   },
+  "c8": {
+    "all": true,
+    "include": ["src/**/*.js"],
+    "reporter": ["text", "lcov"]
+  },
   "dependencies": {
     "d3": "^7.8.0",
     "interval-tree-1d": "^1.0.0",