[rush] Misc fixes for Rush subspaces

[octogonz]

Summary

While working on the Rush subspaces website docs, I found a few issues.

Details

1. If you set subspacesFeatureEnabled=true but are missing common/config/rush/pnpm-config.json then you get extremely confusing error messages such as this:

   Installing for subspace: default
   
   Project filtering arguments can only be used when running in a workspace 
   environment. Run the command again without specifying these arguments.
   

   The reason is that it is a completely untested code path.

   @william2958 This situation is further compounded when common/config/rush/pnpm-config.json is missing, because useWorkspaces defaults to false and is completely ignored in files such as common/subspaces/my-subspace/pnpm-config.json. We need to design a better inheritance model for pnpm-config.json.

2. Further simplify FlagFile internal API.

3. Fix some typos in config files.

4. Add rush init docs for subspaceName

How it was tested

Reproduced using rush-example repo.