[K/JS] Introduce v2015 target

compiler/testData/cli/js/jsHelp.out

@@ -4,7 +4,7 @@ where possible options include:
   -main {call|noCall}        Specify whether the 'main' function should be called upon execution.
   -meta-info                 Generate .meta.js and .kjsm files with metadata. Use this to create a library.
   -module-kind {plain|amd|commonjs|umd|es}
-                             The kind of JS module generated by the compiler.
+                             The kind of JS module generated by the compiler. ES modules are enabled by default in case of ES2015 target usage
   -ir-output-name            Base name of generated files.
   -no-stdlib                 Don't automatically include the default Kotlin/JS stdlib in compilation dependencies.
   -ir-output-dir <directory> Destination for generated files.
@@ -16,7 +16,7 @@ where possible options include:
   -source-map-names-policy {no|simple-names|fully-qualified-names}
                              Mode for mapping generated names to original names (IR backend only).
   -source-map-prefix         Add the specified prefix to the paths in the source map.
-  -target { v5 }             Generate JS files for the specified ECMA version.
+  -target { es5, es2015 }    Generate JS files for the specified ECMA version.
   -Werror                    Report an error if there are any warnings.
   -api-version <version>     Allow using declarations from only the specified version of bundled libraries.
   -X                         Print a synopsis of advanced options.