Core: Throw an error if renderer is used as framework

[IanVS]

Issue:

When upgrading from 6.5 to 7.0, if you just upgrade the versions in package.json instead of running the CLI, and you don't change the framework setting, you'll get an error like this:

Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './preset' is not defined by "exports" in /Users/me/my-project/node_modules/@storybook/react/package.json

It's not clear from this error what the problem is, and it looks like storybook is just broken.

What I did

This PR checks the framework against the list of 6.5 renderers that are not frameworks in 7.0 (angular and ember are frameworks). If one of the "renderers" are found, we will now throw an error that gives a hint of what is actually wrong and how to fix it:

ERR! Error: Invalid value of @storybook/react in the 'framework' field of Storybook config.
ERR! Please see the v7 Migration guide for more information:
ERR! https://github.com/storybookjs/storybook/blob/next/MIGRATION.md#framework-field-mandatory

I went back-and-forth with myself whether I should use a permalink, but I think it's best to have up-to-date docs here, especially since the list of valid frameworks will expand.

Also, I was going to add a test for this, and I found the dev-server.test.ts file which was great, but it turned out to be for a utility function instead, so I moved it. I can remove that commit though if you'd like, since it's not directly related to this PR.

How to test

The way I tested was to build the branch, create a sandbox (linked), and set frameworks to @storybook/react. Running yarn storybook then showed my validation error, instead of the confusing one.

[shilman]

Nice one @IanVS. WDYT about prompting users to upgrade with sb upgrade or to run sb automigrate since those can fix a lot of problems, not just this particular one?

[shilman]

LGTM! 💯