Merge ToC Extension into core

[marthacryan]

References

As discussed in this issue and in the weekly dev meetings, the ToC extension seems to have enough support to be merged into jupyterlab core.

Code changes

To follow the conventions of other core packages, the ToC extension is split into a toc package and a toc-extension package. There are a few changes to the configurations and imports to follow conventions for packages / make the extension work in core. Otherwise it's importing the toc code "as-is" at the moment.

User-facing changes

This will cause the ToC extension to appear as an option in the left side panel along with the File Browser, Running Terminals and Kernels, etc.

[marthacryan]

Fixed the remaining lint errors - I had to use unsafe react methods (such as UNSAFE_componentWillReceiveProps) to fix a couple of these errors, but this method is used in another core extension in Jupyterlab here, so I thought it'd be ok for now - let me know if I should fix that in this PR.

I also changed the version of toc and toc-extension to 4.2.0-alpha.0.

[jasongrout]

Just checking - did you change the collapsing functionality already to a signal or something, or is that something we'll need to do in another PR?

[ajbozarth]

LGTM with the assumption that the follow issues will be opened and dealt with in follow up PRs:

• Update icon usage to LabIcon, this includes moving the svgs into ui-components
• Write tests
• Update Docs to include toc
• Open issues for any other things raised in jupyterlab/team-compass#63 or in key issues on the toc repo

[marthacryan]

@jasongrout No I didn't include the collapsing signal in this PR (it's currently a setting that makes the collapsing disabled by default).

[blink1073]

I opened #8546 for tracking.

[blink1073]

The usage test is hanging again, merging. Thanks @marthacryan! This is an epic first contribution 😄

[marthacryan]

@blink1073 Thank you!