Md list appear

[gksander]

Description

This PR adds a boolean prop animateListItems to the Markdown suite of components that automatically transforms MD list items into Appear > ListItem. This allows a user to provide MD like:

<MarkdownSlide animateListItems>
{`
# Hey world

- One
- Two
- Three
`}
</MarkdownSlide>

and have the list items appear in one at a time.

This addresses issue #968.

Type of Change

Please delete options that are not relevant (including this descriptive text).

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

I have added some Jest/Enzyme tests to test this new prop. I also added an example to the JS example deck that uses this new prop, which can be used to verify that the list items are animating in accordingly.

[gksander]

I'd love to also test for the existence of ListItem components, but those only appear as you step through the slide. I wasn't sure if figuring out how to imperatively do that was worth the time. The enzyme tests are already in the realm of "testing implementation details", so this set of assertions felt sufficient.

In the future, we might consider moving to react-testing-library for testing to move away from testing implementation details. My ideal test would be along the lines of:

• Mount the slide.
• Check that none of the list items are present.
• Simulate a click.
• Check that the first list item is present (searching by text), but not the last two.
• Simulate two more clicks.
• Check that all list items are present (searching by text).

[gksander]

There were some unused imports in this file. I usually clean this up with an editor action for "optimizing imports" (which clears those out). Looks like it also sorts them by alphabetical order, which is why this diff looks bigger than it should. (In effect, all that happened was CodePan and CodeSpan were removed.)

[carloskelly13]

This looks fantastic!