Edit command

[pepicrft]

Related #668

Short description 📝

This PR adds a new command, tuist edit to edit the project in a given directory.

Implementation 👩‍💻👨‍💻

• Update the ProjectEditor utility to raise an error if the directory doesn't contain editable files.
• Implement EditCommand

Testing strategy ✅

• Support generating a project in the same directory without blocking the execution an add an acceptance test that runs the command, and makes sure the targets compile.

[pepicrft]

I followed @kwridan's suggestion and used the -W argument. As he pointed out, the xed's -w option doesn't work as expected.

I also added support for a --permanent argument. When passed, it generates the Manifests.xcodeproj in the directory where the user intends to edit the manifests. The acceptance test that I added uses that argument to generate the project in the same directory and make sure that both, the helpers and the manifests target compile.

[andreacipriani]

leftover or you want to keep the commented code for documentation?

[andreacipriani]

typo ProjectDDescriptionHelpers, is it actually used?