Change default output dir in JS DCE Gradle plugin

See KT-21328

Compiler removes all files from its output directory.
Due to this reason (and some other reasons) it't not a good idea
to put DCE output to `classes` dir.
Therefore we changed output dir to `$buildDir/kotlin-js-min`.
Additionally, we now allow to customize output dir.
In particular, this fixes issues with webpack in multi-project build +
continuous Gradle build.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSDceArguments.kt

@@ -26,6 +26,7 @@ class K2JSDceArguments : CommonToolArguments() {
             valueDescription = "<path>",
             description = "Output directory"
     )
+    @GradleOption(DefaultValues.StringNullDefault::class)
     var outputDirectory: String? by FreezableVar(null)
 
     @Argument(