Register custom file extension for scripts loaded from script template

idea/testData/script/definition/highlighting/customExtension/mainModule/script.mykts

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+1 <error>+</error> "a"

idea/testData/script/definition/highlighting/customExtension/mainModule/template/template.kt

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+package custom.scriptDefinition
+
+import java.io.File
+import kotlin.script.dependencies.*
+import kotlin.script.experimental.dependencies.*
+import kotlin.script.experimental.location.*
+import kotlin.script.templates.ScriptTemplateDefinition
+import kotlin.script.experimental.annotations.KotlinScript
+import kotlin.script.experimental.annotations.KotlinScriptEvaluator
+import kotlin.script.experimental.annotations.KotlinScriptFileExtension
+
+@KotlinScript("Kotlin Script with custom extension")
+@KotlinScriptFileExtension("mykts")
+open class Template