[rush] Rename subspace pnpmfile to .pnpmfile.cjs from .pnpmfile-subspace.cjs

[william2958]

Summary

Design note: https://github.com/microsoft/rushstack/issues/4720

This MR renames the subspace level pnpmfile to ".pnpmfile.cjs" from ".pnpmfile-subspace.cjs", as we are no longer allowing a rush level ".pnpmfile.cjs" file to exist.

This MR also checks if a ".npmrc" file exists globally, and throws an error if it does, prompting the user to use the ".npmrc-global" file instead. It also throws an error if the ".pnpmfile.cjs" file exists at the rush config level.

Details

How it was tested

Impacted documentation