Add design note for converting SUBFLOW to NPM

[HiroyasuNishiyama]

This PR add design note for converting SUBFLOW to NPM module.

Currently Node-RED nodes are distributed as NPM module. There is a design proposal that enable a SUBFLOW to be exported as a node in JSON format (https://github.com/node-red-hitachi/designs/blob/subflow-encryption/designs/exportable-subflow/README.md).

Even if redistributing SUBFLOW in JSON format is possible, it is useful to convert it to NPM format for the following reasons. Therefore, this design note proposes a method of converting JSON-formatted SUBFLOW nodes into NPM modules.

[knolleary]

This is a good start.

Do you have any thoughts on how the runtime then makes use of the subflow json? That is going to be the hard part of this feature and I would like some discussion on the internal changes needed.

[HiroyasuNishiyama]

My assumption is that SUBFLOW definition with registerSubflow is handled through a normal flow import path. I think that we can do the same with a little additional verification.