Combine next.config.js Turbopack options into a nested object (#3689)
This combines Turbopack-specific options into a nested object
`experimental.turbopack`, exposing `loaders` and `resolveAlias`
properties.
x-ref:
https://github.com/vercel/next.js/pull/45560/files/631b637f42757e20aabe84dc933527dc66b55173#r1098123643
Test Plan: Integration tests for loaders and resolveAlias.