Add @jupyterlab/attachments to the build

[jtpio]

References

Fixes #1020

Code changes

Add @jupyterlab/attachments so it ends up in the resolutions and the webpack shared scope.

User-facing changes

This should prevent some JupyterLab extensions depending on the @jupyterlab/attachments package to fail to activate. For example jupyterlab-myst:

Backwards-incompatible changes

[github-actions]

👈 Try it on ReadTheDocs

[bollwyvl]

As noted before: adding a test that compares all the upstream pins/singletons with ours (plus a known list of do-not-wants) would help catch this, and is likely appropriate on this pr.

[jtpio]

As noted before: adding a test that compares all the upstream pins/singletons with ours (plus a known list of do-not-wants) would help catch this, and is likely appropriate on this pr.

Opened #1022 to track this.