Don't require repeating `'split'` when adding ckeditor plugins

## The problem to solve

When adding ckeditor plugins, you need to include `'split'` in the extraPlugins array even though it's included in core already.

## Proposed solution

Apostrophe core should concat the project-level plugins with `['split']` so developers don't need to repeat that core plugin.

## Alternatives

The main alternative would be telling people to include `split` in their project-level configuration.

Start by locating Apostrophe core's CKEditor plugin configuration and the handling of the project-level extraPlugins array. Confirm how the core split plugin is currently included, then verify that combining the configurations keeps split available without requiring projects to repeat it.

Written by the indexing model from the issue text.