Expose NX FsTree for file scaffolding.

[bridzius]

https://github.com/nrwl/nx/blob/master/packages/nx/src/generators/tree.ts and other nx/src utilities are very useful when writing generators/executors that make changes to the workspace.

They are currently not exposed via @nx/devkit, but can be reached by deep importing directly from nx/src/*.

It would be beneficial for NX to expose official "building blocks" via devkit for people to write their own utilities. A similar request has been made in #16691, but it was closed due to inactivity.