Add mocha types

Author: dgreif

package-lock.json

@@ -39,6 +39,7 @@
     "@lhci/cli": "^0.10.0",
     "@open-wc/testing": "^4.0.0",
     "@size-limit/preset-small-lib": "^8.0.1",
+    "@types/mocha": "^10.0.10",
     "@typescript-eslint/eslint-plugin": "^5.36.1",
     "@typescript-eslint/parser": "^5.36.1",
     "@web/dev-server-esbuild": "^0.3.2",