Migrate Jupyter Sidepanel extension to pip-based installation

[Eliaaazzz]

Addresses #37977.

Summary

This PR migrates the Jupyter Sidepanel extension to the pip-based JupyterLab 4
prebuilt installation flow.

What Changed

• add install.json so JupyterLab recognizes the extension as Python-managed
• add style/index.js and styleModule so stylesheet assets are bundled correctly for the prebuilt extension
• make the prebuilt style entrypoint explicitly import the YAML panel styles to avoid ambiguity about whether they are included in the bundle
• update package.json packaging globs so the new style entrypoint is included in published artifacts
• add JupyterLab prebuilt-extension classifiers to pyproject.toml
• include src/yaml/* in tsconfig.json so the YAML panel sources are compiled
• update the sidepanel extension README to document the prebuilt pip install workflow and remove unnecessary legacy jupyter lab build steps
• update the interactive Beam README to replace jupyter labextension install @jupyter-widgets/jupyterlab-manager with pip install ipywidgets

Testing

• jlpm run build:lib
• jlpm run build:prod
• jlpm jest --runInBand
• python -m pip install -e .
• jupyter labextension list
• python -m build --sdist --wheel

Context

JupyterLab 4 favors prebuilt extensions distributed through Python packages instead of the legacy labextension install/link workflow.

This change makes the sidepanel extension discoverable and installable through the expected Python packaging path, and aligns the docs with the supported installation flow.

Checklist

• This PR addresses Migrate Jupyter Sidepanel extension to pip-based installation #37977.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, file an Apache Individual Contributor License Agreement.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request modernizes the installation and packaging of the Jupyter Sidepanel extension by adopting the JupyterLab 4 prebuilt extension standard. By moving away from the legacy 'labextension install' workflow, the extension is now more easily discoverable and installable as a standard Python package. These changes streamline the development and deployment process while ensuring that all necessary assets, including styles, are correctly bundled for the end user.

Highlights

• JupyterLab Extension Migration: Migrated the Jupyter Sidepanel extension to the modern JupyterLab 4 prebuilt extension flow, enabling installation via pip.
• Build and Packaging Configuration: Added 'install.json', updated 'package.json' with 'styleModule' and packaging globs, and included prebuilt-extension classifiers in 'pyproject.toml' to support the new installation method.
• Style Management: Introduced 'style/index.js' as an explicit style entrypoint to ensure YAML panel styles are correctly bundled, and updated 'style/index.css' accordingly.
• Documentation Updates: Updated README files to reflect the new pip-based installation workflow and removed legacy 'jupyter lab build' instructions.

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