Framework main.js field for v7 store

[shilman]

Before:

addons: ['@storybook/react', '@storybook/addon-essentials']

After:

framework: '@storybook/react',
addons: ['@storybook/addon-essentials']

• Add to builder/presets handling
• Add framework: x to CLI for forward-compatibility
• Add error handling (when you don't provide it, or when there's bad exports)
• Add documentation
• Local framework definition?

Frameworks

A framework must provide:

• default render function (can be overridden, currently optional but should be required, see CSF3: Default render functions for non-React frameworks #15498)
• renderToDom function (only specified at the project level)
• applyDecorators function (project level, optional)

In addition can be a preset that provides various other preset functions (webpack, etc.)

[shilman]

Yo-ho-ho!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.4.0-beta.15 containing PR #16393 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

Closing this issue. Please re-open if you think there's still more to do.