Add Indentation to toc

[Jacobzhangjw]

Dear Developer & Maintainer:

It's great to see that jupyterlab also support toc extension now. I tried it but can't help getting to jupyter notebook when editing a long .ipynb.

The reason is that the indentation is not supported in jupyterlab toc, I found sometimes it's hard to look up the toc contents without this visual aids.

Please consider to add this functionality as it works pretty well in jupyter notebook.

[marius311]

I really agree and wish the developers would look at this as its been mentioned before, but in the meantime, you might find the workaround I described in #82 (comment) useful.

[stefansimik]

This is definitely must have for comfortable orientation. I miss the feature too.

[kgryte]

PR submitted to add ToC indentation: #144