add support of exporting subflow module

[HiroyasuNishiyama]

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

Proposed changes

This PR is to realize the export functionality of SUBFLOW module vie Node-RED editor UI.
This is intended to be a draft PR for the following design notes.

• https://github.com/node-red/designs/pull/55
• https://github.com/node-red/designs/pull/26

Checklist

• [x] I have read the contribution guidelines
• [x] For non-bugfix PRs, I have discussed this change on the forum/slack team.
• [x] I have run grunt to verify the unit tests pass
• [x] I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.008%) to 67.668% when pulling bc832651c98b08396e8f95ae093038b4a5a5a615 on node-red-hitachi:export-subflow into f1e7ec0c6bc33d4d0f6286012ef4d5aa817ac0b5 on node-red:dev.

[linux-foundation-easycla[bot]]

The committers are authorized under a signed CLA.

• :white_check_mark: Hiroyasu Nishiyama (bc832651c98b08396e8f95ae093038b4a5a5a615, 5c17a19d6b2bd4a6c23b3486f52e001b9e8f4306)