preliminary GUI support

[k-toumura]

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

Proposed changes

This PR enables an Editor-integrated linter function so that flow developer can validate their code on the fly.

remaining issues

• some rules in browser-side (flowsize) are manually copied from CLI version, and other rules (func-style-eslint, no-func-name) are automatically generated using Webpack. I also plan to use Babel for transpile a modern Node.js code to a code which is compatible with old browsers (IE11, etc.). Use of complex tool lowers the developer's burden (code duplication, browser compatibility issue, etc.), but it also brings complicated build process and larger dependency of other packages. We are still investigating how to write/generate browser-size code.
• http-in-resp and loop rules are not yet ported to Editor-side.
• Some GUI parts (CSS classes, etc.) are borrowed from other part of Node-RED Editor. We should use our own CSS classes.

Checklist

• [x] I have read the contribution guidelines
• [x] For non-bugfix PRs, I have discussed this change on the forum/slack team.

[k-toumura]

The commit in this PR is also included in PR #3. So I close this PR for avoid conflict.