Design: Subflow Node modules

This is a work in progress. Do not start implementing anything without discussing with @knolleary first

The goal of this feature is to allow a subflow to be packaged as an installable npm module and loaded into the runtime.

It will appear in the palette in the same way as any regular node; the end-user will not need to know the node is implemented as a subflow.

Node-RED Nodegen should be updated to supported creating such a module.

Topics this design must cover:

• how the runtime registry knows a module provides a subflow type node
• how the module declares its node dependencies
• how the node's help template and edit template are provided - See also Design: Subflow Instance properties