feat(gatsby-cli): add GATSBY_EXPERIMENTAL_GATSBY_NEW_FLOW flag to use create-gatsby when no options are provided

[gillkyle]

Description

With the new create-gatsby package that doesn't use flags and provides an interactive new experience, this removes the old behavior that asked for a developer to choose a starter first when they used the gatsby new command without any flags or options passed in. The create-gatsby questions are more suited around use cases, instead of having to research what starter you'd want.

This also adds an integration test and changes the docs that mentioned the old interactive behavior.

Note: the create-gatsby package at v0.0.0-6 still has a testing banner on it, so maybe we want to bump the package before we merge this? For that reason I'm making this a draft pull request to start.

Documentation

I updated the gatsby-cli docs to mention this flow instead.

Related Issues

Related to #27703 (once it's merged)

[ascorbic]

Looking good. It should have the testing banner as long as it's still in alpha. This PR can't come into master until create-gatsby is GA, because it changes the default behaviour of gatsby new. A feature flag is one possibility

[mxstbr]

We should add a feature flag for now so we can ship it to make sure it works as expected and this PR doesn't sit around forever! GATSBY_EXPERIMENTAL_GATSBY_NEW_FLOW=true?

[gillkyle]

Thanks for the suggestions 🙂 I just updated to add a GATSBY_EXPERIMENTAL_GATSBY_NEW_FLOW flag.

Screenshot

[ascorbic]

This looks great, and subject to Marvin's suggestions I'm happy to see this go in

[mxstbr]

Love it! 💯

[mfrachet]

go for it! 🚢

[gillkyle]

I really don't think this should affect the CI tests so I'm going to go ahead and merge this since the code is behind a flag anyway and our new release process means it won't even be very publicly facing for a while (even being a Friday afternoon, YOLO) 😬