graphqlbackend: Markdown resolver is string

Created by: keegancsmith

Our Markdown resolver can just be type Markdown string. This will avoid some allocations and simplify interactions with the code. The next few commits will remove the interface and the construction function.

Code in graphqlbackend pkg updated with comby:

comby -in-place '*markdownResolver' 'Markdown' 'go'
comby -in-place '&markdownResolver{text: :[a]}' 'Markdown(:[a])' 'go'