Move MainContentViewSubscriber::WRAPPER_FORMAT somewhere else

Problem/Motivation

MainContentViewSubscriber::WRAPPER_FORMATis referenced in multiple places in core, including a new reference in 🐛 Load pages from pager within active modal Needs work , but it's on an event subscriber which is not supposed to be API. I don't know where we should move it to, but it feels like we could move it somewhere more appropriate.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet