test(next): add tests for Node-like hashbang support

[ctjlewis]

The CLI would break for projects that import from an entry-point with a shebang, though they would be valid Node programs. This PR adds a Webpack loader which removes first-line shebangs from imports to prevent this error.

This approach is safe because emitted bundles will never need a shebang, and even if they did, the line could just be prepended, and you would only ever want one—thus, stripping shebangs from each resolved module is the way to go.

Hashbang logic was added directly to Webpack by @sokra and has been merged into canary. This PR now adds tests for Node-like support of first-line, Unix-style hashbangs, which should prevent regression from the fix for #27806.

[ctjlewis]

Any thoughts on how to address the React Refresh issue as far as the regression test + MJS interop conflict goes?

If the change to that runtime is reverted, the .mjs file will fail to load with the following error:

[ctjlewis]

Even removing the typeof module !== 'undefined' check from the React Refresh logic, tests related to React Refresh are failing.

[ctjlewis]

When Webpack support for hashbangs is merged, this PR will become redundant and can be closed:

webpack/webpack#13961

[styfle]

Thanks for this PR!

Closing in favor of #27929 as you mentioned

[styfle]

Actually, lets reopen this and turn it into just tests

[ctjlewis]

Good idea @styfle - did you see my notes about the .mjs extension conflicting with React Refresh? I force-pushed up with commented versions of the tests. I was only able to work around by adding a typeof module !== 'undefined' check to that runtime, but it broke tests.