Xjdk-release: support mixed compilation of Java and Kotlin

#KT-52815 Fixed

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinCoreApplicationEnvironment.kt

@@ -8,6 +8,7 @@ import com.intellij.DynamicBundle
 import com.intellij.codeInsight.ContainerProvider
 import com.intellij.codeInsight.runner.JavaMainMethodProvider
 import com.intellij.core.JavaCoreApplicationEnvironment
+import com.intellij.ide.highlighter.JavaClassFileType
 import com.intellij.lang.MetaLanguage
 import com.intellij.openapi.Disposable
 import com.intellij.openapi.application.ApplicationManager
@@ -26,6 +27,10 @@ class KotlinCoreApplicationEnvironment private constructor(
 ) :
     JavaCoreApplicationEnvironment(parentDisposable, unitTestMode) {
 
+    init {
+        registerFileType(JavaClassFileType.INSTANCE, "sig");
+    }
+
     override fun createJrtFileSystem(): VirtualFileSystem {
         return CoreJrtFileSystem()
     }

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/modules/CtSymClassVirtualFile.kt

@@ -5,6 +5,9 @@
 
 package org.jetbrains.kotlin.cli.jvm.modules
 
+import com.intellij.ide.highlighter.JavaClassFileType
+import com.intellij.openapi.fileTypes.FileType
+import com.intellij.openapi.fileTypes.FileTypeRegistry
 import com.intellij.openapi.vfs.VirtualFile
 import java.io.InputStream
 
@@ -39,4 +42,8 @@ class CtSymClassVirtualFile(
     override fun refresh(p0: Boolean, p1: Boolean, p2: Runnable?) = file.refresh(p0, p1, p2)
 
     override fun getInputStream(): InputStream? = file.inputStream
+
+    override fun getFileType(): FileType = JavaClassFileType.INSTANCE
+
+    override fun getModificationStamp(): Long = file.modificationStamp
 }