Fix NPM7 peer dependencies

NPM7 has changed the semantics of peer dependencies which caused NPM to fail: #12983

I updated the CLI to use --legacy-peer-deps to work around this issue: #14106

However, the underlying structure of the project has not changed, and presumably this will bite us in the future.

There are at least two kinds of dependency errors:

• Dependencies on external libraries like @reach/router which are not React17 compatible
• Use of optional peer dependencies within Storybook itself, e.g. addon-docs

External libraries

For each library that is not NPM7 compatible, we should either

• Fork it, make it compatible, issue a PR, replace the fork with the upgrade once PR is merged
• OR upgrade to an alternative

In the case of @reach/router, the path forward could be React Router which does support React 17

Internal dependencies

@storybook/addon-docs has optional peer dependencies on each of the framework packages it supports @storybook/react, @storybook/vue, etc. It should be refactored to use dependency injection so that those dependencies can be removed. In addition, the react-specific code should probably be moved into @storybook/react, which is a bigger project.

[ndelangen]

We recently upgraded to react-router v6!!

[Jordan-Hall]

upgrade to react-router 6 but that requires some non-trivial refactoring.

Ok so gonna try and starat this then.