[cli] Add `sanity init --bare` flag

[rexxars]

Type of change (check at least one)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (fix or update to documentation)
• [ ] Maintenance
• [ ] Other, please describe:

Does this change require a documentation update? (Check one)

• [x] Yes
• [ ] No

Description

This PR allows a user to run sanity init --bare and only create a project and (optionally) a dataset. If the user chooses to, it will also create a folder with a sanity.json (containing project ID and dataset) and a package.json (containing @sanity/core as a dependency).

I didn't want to create even more "branches" in the init project code which is already a bit unwieldy, so I separated some logic into their own files for reuse between the "init bare" method and the "regular" init.

Note for release

Add --bare flag to CLI sanity init command, for cases where a studio is not needed.

Checklist

• [x] I have read the Contributing Guidelines
• [ ] The PR title includes a link to the relevant issue
• [x] The PR title is appropriately formatted: [some-package] PR title (#123)
• [x] The code is linted
• [x] The test suite is passing
• [ ] Corresponding changes to the documentation have been made

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[rexxars]

Will revisit this later.