[rush] Adds support for a separate rush-projects.json configuration file

[william2958]

Summary

This MR adds the optional rush-projects.json file that allows the rush projects to be defined in a separate, dedicated file. This is useful in situations where repo permissions are managed at a file-level, allowing more developers to add or remove projects without admin repo level permissions.

Details

This change will change the way rush looks for projects. It will first look for a rush-projects.json file, then if it does not exist it will look within the rush.json file, same as before. It prevents projects from being defined in both places, and also ensures that the projects are defined in one of the two locations.

How it was tested

Tested locally on a rush repo.

Impacted documentation

https://rushjs.io/pages/configs/rush_json/