Reuse some parts from @jupyterlab/builder

[jtpio]

For now many files are copied from https://github.com/jupyterlab/jupyterlab/tree/main/builder, and modified for use with Rspack.

While this package is still meant to be a temporary solution while we work on integrating this in core JupyterLab, maybe it could still be worth looking into reusing some parts of the upstream package. For example metadata_schema.json.