update list environments to internal_vertical

[dginev]

This is a first step towards addressing the mixed-mode lists that are encountered when dealing with latex's itemization environments.

• The main contribution is declaring them all as mode => "internal_vertical"
• \preitem@par seemed to have an inaccuracy (I think) in checking the current element name is not an itemization element.
• interestingly, the final digested \par doesn't really seem needed for our tests to pass. The autoOpen/autoClose schema guarantees may be sufficient to deal with the markup side.

What is still missing here is ensuring the horizontal material of the last item gets repacked at an itemization's end.

[brucemiller]

Other than a few commands missing mode, it appears the main thing going on is avoiding digesting the \par. Curious. But the tests pass.

[dginev]

Thanks for the review, I added the corrections.

So far it appears that this changeset is somewhat stable (no visible regressions) over arXiv. Plenty of examples where the missing part is still biting us though -- namely very tall heights for certain complex {tcolorbox} contents. I am yet to cleanly solve the "mixed mode" details for Lists of Boxes.