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/.eslintrc.js

@@ -0,0 +1,13 @@
+module.exports = {
+  rules: {
+    'no-restricted-imports': [
+      'error',
+      {
+        /** The mocha tests run on the compiled output in the /lib directory
+         * so we should avoid importing from src.
+         */
+        patterns: ['*src*'],
+      },
+    ],
+  },
+};