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/ignorehttpserrors.spec.ts

@@ -19,7 +19,7 @@ import {
   getTestState,
   describeFailsFirefox,
   itFailsFirefox,
-} from './mocha-utils';
+} from './mocha-utils'; // eslint-disable-line import/extensions
 
 describe('ignoreHTTPSErrors', function () {
   /* Note that this test creates its own browser rather than use