chore: enforce file extensions on imports (#6202)

* chore: enforce file extensions on imports

To make our output agnostic it should include file extensions in the
output, as per the ESM spec. It's a bit odd for Node packages but makes
it easier to publish a browser build.

test/coverage.spec.ts

@@ -20,7 +20,7 @@ import {
   setupTestPageAndContextHooks,
   setupTestBrowserHooks,
   describeChromeOnly,
-} from './mocha-utils';
+} from './mocha-utils'; // eslint-disable-line import/extensions
 
 describe('Coverage specs', function () {
   describeChromeOnly('JSCoverage', function () {