Support custom loaders in ESM format

[eirikur-nc]

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

• 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:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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