Update plugin compatibility with JupyterLab 3.0 and release 0.4.0 version

[Dominos]

We've upgraded the extension following by https://jupyterlab.readthedocs.io/en/stable/extension/extension_migration.html

It's been tested and works as expected

[Dominos]

Hey @mikehcheng, may I ask you to review the PR?

[kbroughton]

would be great if you could merge this and push a new pip version

[Dominos]

@kbroughton actually we've built our internal CI/CD pipeline and publish in our internal artifactory.

If you want here's a sample to build this package on Github Actions:

name: CI

on: [push, workflow_dispatch]

jobs:
  Build:
    runs-on: [ self-hosted, ubuntu ]
    steps:
      - name: Checkout source code
        uses: actions/checkout@v2

      - name: setup npm & nodejs
        uses: actions/setup-node@v2
        with:
          node-version: '16'

      - name: setup Python3.8 environment
        uses: actions/setup-python@v2
        with:
          python-version: '3.8'

      - name: install build dependencies
        run: python3.8 -m pip install -r build_requirements.txt

      - name: build package
        run: python3.8 -m build

[mimizone]

checking if somebody is still considering merging this and making it available to the labextension repo