Get self reference modules working

I was partially confused due to a potential spec bug
in package.exports that didn't take into account top-level
application (without a . main)

This now works and has a passing test.

Refs: nodejs/node#30633

.gitignore

@@ -0,0 +1 @@
+.nyc_output/

lib/constants.mjs

@@ -0,0 +1,8 @@
+import { dirname, resolve } from 'path';
+import { fileURLToPath } from 'url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+/** @type {string} */
+export const projectRoot = resolve(__dirname, '../');