configuring and adding sample widgets

[edublancas]

we're gonna start building some widgets, this PR:

• the necessary configuration
• added some basic tests
• cleaned up README files
• added two example widgets (in the _widgets private module)

there are still a few pending improvements that I'll tackle in: #19

[neelasha23]

Added some comments.
We can add a developers guide that outlines the current scope, goals/ use cases, future work, etc.

[yafimvo]

Added some notes.

General notes:

1. It takes a lot of time to run pip install -e ".". At first, I thought my setup was stuck so I restarted it. (Windows 11 - wsl2). Maybe we can point it out.
2. I think jlpm watch is not working for me. Should it automatically rebuild the plugin after saving? Currently, I run jupyter lab and on every change in the plugin's code, I restart the Kernel.
3. I wasn't able to run the tests:

   • I ran nox --session test - took ~50min to install the env and complete the tests. Logs and errors are attached:
     nox-output.txt

   • I ran jlpm test - after ~15 min it failed. Logs and errors are attached:
     jlpm_test-output.txt

   • After running each test, I ran npx playwright show-report and it showed nothing
     

[edublancas]

Added some comments. We can add a developers guide that outlines the current scope, goals/ use cases, future work, etc.

good point. opened: #21

[edublancas]

Added some notes.

General notes:

1. It takes a lot of time to run `pip install -e "."`. At first, I thought my setup was stuck so I restarted it. (Windows 11 - wsl2). Maybe we can point it out.

added a note

2. I think `jlpm watch` is not working for me. Should it automatically rebuild the plugin after saving? Currently, I run `jupyter lab` and on every change in the plugin's code, I restart the Kernel.

are you seeing new messages in the terminal? if so, it's working. if the change is in python, you need to restart the kernel (or use the IPython autoreload feature). If you change the frontend code, you need to refresh the browser tab.

3. I wasn't able to run the tests:
   
   * I ran `nox --session test` - took ~50min to install the env and complete the tests. Logs and errors are attached:
     [nox-output.txt](https://github.com/ploomber/jupysql-plugin/files/11589813/nox-output.txt)
   * I ran `jlpm test` - after ~15 min it failed. Logs and errors are attached:
     [jlpm_test-output.txt](https://github.com/ploomber/jupysql-plugin/files/11589580/jlpm_test-output.txt)
   * After running each test, I ran `npx playwright show-report` and it showed nothing
     ![image](https://user-images.githubusercontent.com/103026689/241675922-29680cda-5ca6-481c-aa8a-6ed050c02449.png)

could this be an OS issue? the command is running in the CI on mac and it works fine:

jupysql-plugin/.github/workflows/ci.yaml

Line 53 in fddbfba

nox --session test --no-install --reuse-existing-virtualenvs

I opened an issue so we test in other OSs: #19

ready to merge