Combine two RouterProvider implementations

[ryanflorence]

• We started with <RouterProvider> that lived in react-router, and worked for memory/dom routers
• When we added flushSync and viewTransition stuff, those are DOM APIs so we had to make a new <RouterProvider> that only works in react-router-dom
• With v7 there is only react-router-dom (but it's react-router), so we only need one <RouterProvider>

This task is to end up with a single implementation.

Questions

We need to decide if we care about transitions checking for DOM APIs like view transitions and react's flushSync, particularly for people who have unit tests running w/ memory routers, etc. We can probably still support them by being defensive about the DOM/React APIs that wouldn't work outside of a DOM.

[brophdawg11]

#11679