Pr/fix importing mdx from node modules

[gaelhameon]

What: return undefined instead of {} when inMemoryPlugin doesn't handle the file, to actually let esbuild (or other plugins) handle the file. Solves #41

Why: the esbuild doc is kind of confusing, but empirical evidence suggests that returning {} doesn't allow other plugins or esbuild itself to try and resolve/load the file

How: replace {} by undefined where necessary

Checklist:

• Documentation N/A
• Tests
• Ready to be merged

[Arcath]

Thanks for the PR!

I've implemented this change in #45 along with some other improvements for #41. I didn't see that you where going to opena PR until I'd already done the work.

I added you as a co-author on the first commit as the issues with resolution where solved using your suggestions in #41 before I moved the compiling into its own esbuild plugin.

[Arcath]

This has been fixed by #45