Fix writeConfigurationDefaults.ts to correctly suggest changes in monorepos (#48668)
Closes #48653. It should either modify the TS config to add the Next.js
plugin, or output a log if it can't (to avoid overriding the base
config).
<img width="920" alt="CleanShot-2023-04-21-Klqel8BO@2x"
src="https://user-images.githubusercontent.com/3676859/233642532-5eed8d5d-d2c5-41e9-99bf-d0eb95685206.png">