eslint: Unambiguous ES module rule

Currently, Script and Module goals have a grammatical ambiguity where some code can run in both goals, having the exact same source text, but produce different results. Unlike "use strict", the signal to have a specific behavior is not in the code, thus the code has a multitude of possible effects which are not controlled by the programmer.

Because of this I think a new rule for unambiguous ES modules would be nice. It could even be enabled by default when "parserOptions": { "sourceType": "module" }. The rule would enforce that an ES module should have at least one import or export declaration.

\cc @mikesherov @ilyavolodin