rule to detect non-es6module import

As the es6 is widely used and some libraries are already distributed as es6 (I hope there will be more of them in future) it will be nice to have rule to check if the imported file contains es6 module or not.

I see the biggest benefits of es modules in predictability and better possibilities during static analyses. It can be used for better code optimisation - tree shaking, death code elimination, ...

No file, test, or entry point is named. Start by reviewing the plugin's existing import rules and how they inspect imported files, then clarify the expected ES6-module detection behavior and add coverage; done means the rule reliably identifies whether an imported file contains an ES6 module.

Written by the indexing model from the issue text.