Prepare for Workbench Beta Phase (update styles)

[zkamvar]

Hello @LibraryCarpentry/lc-shell-maintainers,

This PR does two things:

1. it brings in the updates from styles from 2018 to present
2. 6807b96f674764469047346f435ba74ee44f6617 updates the lesson code block highlighting to comply with the changes from https://github.com/carpentries/styles/pull/498
   • impetus: see https://github.com/carpentries/styles/pull/496#issuecomment-676482224
   • original implementation: https://github.com/carpentries/styles/pull/202

Merging this will allow me to work on #211

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[tobyhodges]

@LibraryCarpentry/lc-shell-maintainers this PR is currently blocking the lesson's entry into the Workbench beta phase (#211) - please let me or @zkamvar know if you would like any help reviewing and/or merging it. It is very large and I could definitely understand if the task seems somewhat daunting!

(Incidentally, one of the big advantages of switching over to the new lesson infrastructure will be that large pull requests like this to update lesson site styling and repository mechanics will no longer be necessary.)

[zkamvar]

I have updated this pull request to also include the necessary information for the workbench beta phase.

[jmjamison]

merging to prepare for new workbench beta