Add Modern WWW build (#18028)

* Build both stable and experimental WWW builds

* Flip already experimental WWW flags to true

* Remove FB-specific internals from modern FB builds

We think we're not going to need these.

* Disable classic features in modern WWW builds

* Disable legacy ReactDOM API for modern WWW build

* Don’t include user timing in prod

* Fix bad copy paste and add missing flags to test renderer

* Add testing WWW feature flag file

We need it because WWW has a different meaning of experimental now.