[Gradle, JS] Make addNpmDependencyExtension as internal

#KT-38683 fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/npm/NpmDependencyExtension.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -92,7 +92,7 @@ private class DefaultNpmDependencyExtension(
     }
 }
 
-fun Project.addNpmDependencyExtension() {
+internal fun Project.addNpmDependencyExtension() {
     val extensions = (dependencies as ExtensionAware).extensions
 
     NpmDependency.Scope.values()