convert repo to monorepo #1910

The repository is a mess currently.

The below changes can be made without becoming breaking changes.

new structure:

> packages
  > utilities # all utilities that aren't sortable
  > plugins # all the plugins
    > auto-scroll
    > on-spill
    > multi-drag
    > swap
  > sortable # the Sortable class and friends. Friends may be split up later.
  > sortablejs # the entrypoints: default, core, complete

lerna seems to be a good choice because by default it bumps the version of all packages simultaenously when only one package has been changed. It ensures compatibility between all plugins and sortable.

This will help the development for version two of sortable.