"Avoid fake ES6 syntax" rule

[jbmoelker]

Summary

Proposed changes:

• "Avoid fake ES6 syntax" rule (incl. summary, why, how and examples).

  Checklist

• [X] The changes only affect or contain one style guide rule or section.
• [X] The changes are in style guide format.
• [X] The new style guide section has an entry in the Table of Contents (only if changes introduce new section).
• [x] The changes can be merged into the target branch without conflicts.

[jbmoelker]

@thadeetrompetter d158263 fixes https://github.com/voorhoede/riotjs-style-guide/pull/22#discussion-diff-54862147 . Any other remarks? Or would you approve?