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/headful.spec.ts

@@ -23,7 +23,7 @@ import {
   getTestState,
   describeChromeOnly,
   itFailsWindows,
-} from './mocha-utils';
+} from './mocha-utils'; // eslint-disable-line import/extensions
 import rimraf from 'rimraf';
 
 const rmAsync = promisify(rimraf);