Make list element title and content fields align with body in Composer

[rhystmills]

What does this change?

Makes some minor tweaks to nestedElement field and altStyle wrapper to make sure the list element title and content align with the main editor text in Composer. They were once aligned but there has been some drift.

Before	After

How to test

1. Use yarn yalc in this repo to publish prosemirror-elements locally.
2. Use it in the composer directory of flexible-content with yalc add @guardian/prosemirror-elements
3. Create one of the list elements and check out the alignment.

[simonbyford]

This was very jarring, thank you