Include @jupyterlab/builder in this package

[fcollonval]

We should bring @jupyterlab/builder within this package and make it generic.

For now the code lives there: https://github.com/jupyterlab/jupyterlab/tree/main/builder

[jtpio]

After the move, should the package still be named @jupyterlab/builder, or @jupyter/builder?

Since the Python package seems to be named jupyter_builder at the moment:

https://github.com/jupyterlab/jupyterlab-builder/blob/c04e4a2523df1ccbfc2d4bf9edb34cdd2bd25fb3/pyproject.toml#L9

Looks like @jupyterlab/builder may be preferable to avoid changing extensions that depend on it. Although there might be some ways to handle renames of npm packages without too much disruption.

[fcollonval]

@cronan03 to test this, you need to

1. Bring the Typescript files and the package.json
2. Build the Javascript assets; normally it should create a lib folder when you execute jlpm run build
3. You should check that the code is calling those Javascript assets in that Python function:

https://github.com/jupyterlab/jupyterlab-builder/blob/c04e4a2523df1ccbfc2d4bf9edb34cdd2bd25fb3/jupyter_builder/federated_extensions.py#L215

That implies defining properly the core_path to get the correct builder variable (it should point to the lib/build-labextension.js file)