Introduce Kotlin language substitutor to fix exceptions in velocity files

Related to KT-31470 (should fix the problem or at least soften it)
May be related also to KT-30977
#KT-30886 Fixed

idea/resources/META-INF/plugin-common.xml

@@ -863,6 +863,8 @@
 
     <orderEnumerationHandlerFactory implementation="org.jetbrains.kotlin.idea.roots.KotlinNonJvmOrderEnumerationHandler$Factory"/>
 
+    <lang.substitutor language="kotlin" order="last" implementationClass="org.jetbrains.kotlin.idea.KotlinLanguageSubstitutor"/>
+
     <intentionAction>
       <className>org.jetbrains.kotlin.idea.intentions.FoldInitializerAndIfToElvisIntention</className>
       <category>Kotlin</category>

idea/src/org/jetbrains/kotlin/idea/KotlinLanguageSubstitutor.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2010-2019 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.
+ */
+
+package org.jetbrains.kotlin.idea
+
+import com.intellij.lang.Language
+import com.intellij.openapi.fileTypes.PlainTextLanguage
+import com.intellij.openapi.project.Project
+import com.intellij.openapi.vfs.VirtualFile
+import com.intellij.psi.LanguageSubstitutor
+
+class KotlinLanguageSubstitutor : LanguageSubstitutor() {
+    override fun getLanguage(file: VirtualFile, project: Project): Language? {
+        val name = file.name
+        if (name.endsWith(".kt.ft")) {
+            return PlainTextLanguage.INSTANCE
+        }
+        return null
+    }
+}