refactor: generic dev build indicator (#74615)
- Decoupled pages router specific logic from the dev build indicator to
extract a generic version
- Created a pages router specific initialize function for dev build
indicator to be called on pages router hydration
- This prepares for upcoming work on the dev build indicator for App
router
- Covered by [e2e
test](https://github.com/vercel/next.js/blob/jude/refactor-dev-build-indicator/test/integration/build-indicator/test/index.test.js)
