Support *.md for Turbopack

[gaearon]

This is maybe not the right way to do it but I've wasted half an hour on this.

The docs here imply that .md extension will work with Turbopack but it absolutely does not without something like this.

Alternatively, the misleading docs need to change to not list .md extension at all.

I have not actually tested the PR, this is just a guess

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 41d51e0

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[wbinnssmith]

The docs here imply that .md extension will work with Turbopack but it absolutely does not without something like this.

Yeah, this is misleading. Right now it's impossible to configure @next/mdx with Turbopack to use anything but .mdx, and it's not even the default with webpack. The challenge with this particular change is that it makes Turbopack behave differently than webpack by default.

While with webpack you can configure extensions as a file path regex, we don't support that with Turbopack. We're working out a way to achieve a similar result while being backwards compatible. Should get back to you soon.

[wbinnssmith]

@gaearon, I've got a PR to address this with #78563, let me know what you think.