pptx: Add support for incremental lists

- Support -i option
- Support incremental/noincremental divs
- Support older block quote syntax
- Add tests

One thing not clear from the manual is what should happen when the input
uses a combination of these things. For example, what should the
following produce?

```md
::: {.incremental .nonincremental}
- are
- these
- incremental?
:::

::: incremental
::::: nonincremental
- or
- these?
:::::
:::

::: nonincremental
> - how
> - about
> - these?
:::
```

In this commit I’ve taken the following approach, matching the observed
behaviour for beamer and reveal.js output:

- if a div with both classes, incremental wins
- the innermost incremental/nonincremental div is the one which takes
  effect
- a block quote containing a list as its first element inverts whether
  the list is incremental, whether or not the quote is inside an
  incremental/non-incremental div

I’ve added some tests to verify this behaviour.

This commit closes issue #5689
(https://github.com/jgm/pandoc/issues/5689).

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