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).

1 files changed, 0 insertions, 3 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index b65e45bfe..82fc21684 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -5919,9 +5919,6 @@ option):
 Both methods allow incremental and nonincremental lists to be mixed
 in a single document.
 
-Note: Neither the `-i/--incremental` option nor any of the
-methods described here currently works for PowerPoint output.
-
 ## Inserting pauses
 
 You can add "pauses" within a slide by including a paragraph containing