Support custom loaders in ESM format

Description

Use a dynamic import when loading custom loaders in order to support loaders in ESM format. Retain the original implementation, which relies on createRequire, for the synchronous version useCustomLoaderSync

Related #6656

Type of change

[x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I could not reproduce the issue using Jest, as it transpiles esm to commonjs.

To keep transpilation to a minimum, I added a test that can be run using the Node.js test runner with type-stripping enabled (requires version 22.6 or later). I also added a convenience script to the root package.json file so that the test can be run using yarn test:node

I realize that having to use two test runners is not ideal. I'm open to suggestions if there's a simpler way to demonstrate the effectiveness of this fix.

Checklist:

[x] I have followed the CONTRIBUTING doc and the style guidelines of this project
[x] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[ ] My changes generate no new warnings
[x] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests and linter rules pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules

ardatan / graphql-tools