chore(deps): bump postcss and @jupyterlab/builder in /js

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.49 and updates ancestor dependency [@jupyterlab/builder](https://github.com/jupyterlab/jupyterlab). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.4.49
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.49)

Updates `@jupyterlab/builder` from 3.0.0-rc.6 to 4.3.4
- [Release notes](https://github.com/jupyterlab/jupyterlab/releases)
- [Changelog](https://github.com/jupyterlab/jupyterlab/blob/@jupyterlab/[email protected]/CHANGELOG.md)
- [Commits](https://github.com/jupyterlab/jupyterlab/compare/@jupyterlab/[email protected]...@jupyterlab/[email protected])

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@jupyterlab/builder"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>