Split Route Modules

[brophdawg11]

RFC: #11571
PR (unstable): #11871

From Mark in Discord:

unstable_splitRouteModules could be stabilised easily, we might just need to chat about the API. My proposal would be to have it enabled by default, but allow splitRouteModules as a top-level option so you can set it to enforce mode or disable it entirely (in case the code transform is causing any issues). These are the current set of options on the future flag, it's just flipping the default to true.