Add ES6 compatibility to CKBuilder

[Wiktor Walc]

Steps to reproduce

Upload addon with ES6 syntax (e.g. let instead of var) to the addons repo.

or even better:

Create a plugin with ES6 syntax and use command line builder to build CKEditor.

Expected result

Everything should work.

Actual result

Validating JS files plugin.js (line 19):

missing ; before statement

ES6_FEATURE. this language feature is only supported in es6 mode: let declarations. Use --language_in=ECMASCRIPT6 or ECMASCRIPT6_STRICT to enable ES6 features. at .tmp803549/plugin.js line 19 : 12

Other details (browser, OS, CKEditor version, installed plugins)

[Marek Lewandowski]

Not a high priority for now unless our community will actively request it in comments.