[Proof of Concept]: Introduce Flowtype

[tetsuharuohzeki]

https://flowtype.org/

TODO

• lint
  • https://github.com/gajus/eslint-plugin-flowtype
  • https://github.com/amilajack/eslint-plugin-flowtype-errors
• editor integration
• npm run command integration
• import type definitions

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This change is

[dokidokivisual]

☔ The latest upstream changes (presumably #815) made this pull request unmergeable. Please resolve the merge conflicts.