Add factory token for jupytext customization

[fcollonval]

This:

• will improve tree shaking when only requiring the token.
• add a new token to customize the widget factory (for use in Jupytext)

Fixes partly #17 (comment) cc @nthiery

This will need to be backported on 3.x and then a PR will be required on jupytext itself (code already written and tested https://github.com/fcollonval/jupytext/tree/ft/support-lab-rise).

[github-actions]

👈 Launch a Binder on branch jupyterlab-contrib/rise/enh/create-token-file

[fcollonval]

@meeseeksdev please backport to 3.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 3.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 e8dedca3690c3b1d0812d18bc23c9febb24cd925

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #50: Add factory token for jupytext customization'

4. Push to a named branch:

git push YOURFORK 3.x:auto-backport-of-pr-50-on-3.x

5. Create a PR against branch 3.x, I would have named this PR:

"Backport PR #50 on branch 3.x (Add factory token for jupytext customization)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[fcollonval]

Jupytext PR is up: mwouts/jupytext#1126