Adopt Yarn workspaces/lerna and separate out the demo and library "proper"

[charlie-yao]

• Adopt a monorepo approach by using yarn workspaces and lerna
• Separate the repo into two yarn workspaces/lerna packages - react-aria-widgets for the library "proper" and react-aria-widgets-demo one for the demo/docs website
• Begin using React Storybook on the accordion components in react-aria-widgets
• Webpack config changes
  • separate dev and prod configs
  • begin splitting up code into separate chunks
• Misc. changes, e.g.:
  • ESLint config modifications

Resolves #11
Related to #8, #9

https://yarnpkg.com/features/workspaces
https://github.com/lerna/lerna
https://mytechblog.io/lerna-yarn-workspaces
https://leerob.io/blog/monorepo-lerna-yarn-workspaces
https://verdaccio.org/blog/2019/09/07/managing-multiples-projects-with-lerna-and-yarn-workspaces/
https://www.honeybadger.io/blog/monorepo-yarn-workspace-lerna/

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Note: added core-js as a dev dependency in the root workspace because it fixes an issue where react-storybook complains:

ERR! Error: @storybook/addon-interactions tried to access core-js, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.    

There's probably a better fix or workaround somewhere in storybookjs/storybook#11255, but this one seems to work well-enough and doesn't seem to add extra clutter to PnP cache.