Logistics and tasks for version 6

[lyzadanger]

This is a grab-bag issue of tasks related to version 6 of this project's package.

Remove legacy assets

See also #566

These tasks are related to removal of legacy components, assets and support, and associated cleanup tasks.

• Remove legacy components
• Rename "Next" component functions and exports
• Update test.css test CSS entrypoint to remove legacy styles
• Update templates in plop-templates:
  • Remove references to "how to migrate" in pattern library template
  • Update naming and exports to remove "Next"
• Remove legacy component documentation
• Ensure all legacy CSS/SASS is removed and update SASS/CSS README
• Remove support for clean-theme styling (this should be local to the client application)
• Remove icon registration from pattern library
• Remove custom importOrder regexp from package.json once legacy components are removed (see comment)
• Update developing.md content
• Check for internal dependencies on next entrypoint
• Update Usage component in Library (for entrypoint path)
• Update any paths referencing next in the pattern library docs, e.g. Getting Started page
• Update all relevant pattern-library pages

Package Housekeeping

• Export types from package entry point (See Export types in some sort of sane way #858)
• Ensure all needed exports are in lib/next (including, e.g., hooks)
• Replace the current entrypoint with (contents of) lib/next? Deprecation of lib/next
• Review peer dependencies
• Pattern library: review API

Tasks associated with the removal of legacy components and assets can be found in this issue.