feat!: switch to native ESM with NodeNext module resolution and .js import paths

BREAKING CHANGE: Consumers must use ESM-compatible environments.
All import paths now include .js extensions.
CommonJS (require) is no longer supported.

src/functions/register.ts

@@ -1,5 +1,5 @@
-import { TSinjex } from '../classes/TSinjex';
-import { Identifier } from '../types/Identifier';
+import { TSinjex } from '../classes/TSinjex.js';
+import { Identifier } from '../types/Identifier.js';
 
 /**
  * Register a dependency.