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/mocha-ts-require.js

@@ -0,0 +1,8 @@
+require('ts-node').register({
+  /**
+   * We ignore the lib/ directory because that's already been TypeScript
+   * compiled and checked. So we don't want to check it again as part of running
+   * the unit tests.
+   */
+  ignore: ['lib/*', 'node_modules'],
+});