Re-target for ES.Next (#3901)

A list of changes:
- 'es2015-subset' profile is deprecated, and an 'es.next' profile is added.
- The default profile is changed to 'es.next'
- Renamed the JERRY_ES2015 guard to JERRY_ESNEXT
- Renamed JERRY_ES2015_BUILTIN_* guards to JERRY_BUILTIN_*
- Moved es2015 specific tests to a new 'es.next' subdirectory
- Updated docs, targets, and test runners to reflect these changes

Resolves #3737.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

docs/15.MODULE-SYSTEM.md

@@ -2,7 +2,7 @@
 
 The module system allows users to write import and export statements in scripts, which can be used to separate the logic of the application into custom modules.
 The standard's relevant part can be found [here](https://www.ecma-international.org/ecma-262/6.0/#sec-modules).
-Embedders wishing to use native builtin modules with ES6 imports can use the [Port API](05.PORT-API.md#es2015-module-system) to do so.
+Embedders wishing to use native builtin modules with ES6 imports can use the [Port API](05.PORT-API.md#jerry-module-system) to do so.
 
 ## General
 
@@ -76,7 +76,7 @@ export var version = "1.0 (e92ae0fb)";
 
 ```js
 // module_2.js
-var featureName = "EcmaScript 2015 modules";
+var featureName = "EcmaScript modules";
 var year = 2018;
 
 export function getFeatureDetails() {