Presentations: Use divs to set incremental/non-incremental

Currently, html and beamer presentations use a list-inside-blockquote
convention for setting incremental and all-at-once presentation of
lists (or reversing the command-line default). This allows the user to
set this on a per-case basis with divs, named `incremental` and
`nonincremental` respectively, as in:

    ::: incremental

    - snap

    - crackle

    - pop

    :::

Note that the former list-inside-blockquote convention still works so
as not to break any existing presentations.

Closes: #4381

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