Add preprocess hook to add layout link to Inline entity form

Patch to add this requested functionality.

I think we need to create a MercuryEditorTasksInlineEntityForm service for just the preprocess hook.

Here are the steps to setup a site to expose this functionality

1. Enable mercury editor and inline entity form
2. Add a new content type
3. Add a layout paragraphs field
4. Enable that content type with mercury editor
5. Create a paragraph
6. In that paragraph, add a content reference field that references nodes
7. Have its reference use the content type you created above, save
8. Edit the new paragraphs' form mode
9. For the content reference field in this paragraph change the form mode to 'Inline entity form - complex'
10. Go back to your content type and add this paragraph to the layout paragraphs field
11. Create a new piece of content (of the type you created in step 3)
12. You just need to title it, you don't need to add any other content, save
13. Create another piece of content of the same type
14. Edit the layout of that new piece of content
15. Add a paragraph of the type you created in step 6
16. In the content reference field of this paragraph, select 'add an existing node'
17. Search for the piece of content you created in step 12 (it should probably be the only one in the list) and select it
18. After the entity select modal goes away there should now be another button after 'edit' that says 'Layout'
19. That 'layout' button in the inline entity form is what this patch adds.