Implement simple "add to compilation" functionality for scripts

(incomplete import - scripts added to the compilation but not yet
imported into scope)
Base functionality needed for #KT-27960

libraries/scripting/common/src/kotlin/script/experimental/api/scriptCompilation.kt

@@ -77,6 +77,11 @@ val ScriptCompilationConfigurationKeys.providedProperties by PropertiesCollectio
  */
 val ScriptCompilationConfigurationKeys.defaultImports by PropertiesCollection.key<List<String>>()
 
+/**
+ * The list of script sources that should be compiled along with the script and imported into it
+ */
+val ScriptCompilationConfigurationKeys.importScripts by PropertiesCollection.key<List<SourceCode>>()
+
 /**
  * The list of script dependencies - platform specific
  */