[FIR] Add FirExtensionRegistrarAdapter to fix extension points in FE 1.0 IDE plugin

FirExtensionRegistrar is FIR specific extension which requires FIR
  classes to work, which are missing in dependencies of FE 1.0 IDE plugin.
  To fix this issue FirExtensionRegistrarAdapter is introduced. This
  class is a base class for FirExtensionRegistrar and it does not have
  any FIR specific classes in it's API, so it can be safely registered
  in IDE

compiler/frontend.common/src/org/jetbrains/kotlin/fir/extensions/FirExtensionRegistrarAdapter.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2021 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.fir.extensions
+
+import org.jetbrains.kotlin.extensions.ProjectExtensionDescriptor
+
+/*
+ * This is needed to prevent ClassNotFoundExceptions in IDE plugin built on FE 1.0
+ *
+ * Actual FirExtensionRegistrar uses some specific classes from FIR, so if we reference
+ *   it in KotlinCoreEnvironment (where all extension points are registered) then we
+ *   need to have all FIR compiler on classpath. And in IDE plugin we don't depend
+ *   on FIR to reduce plugin size.
+ */
+abstract class FirExtensionRegistrarAdapter {
+    companion object : ProjectExtensionDescriptor<FirExtensionRegistrarAdapter>(
+        name = "org.jetbrains.kotlin.fir.extensions.firExtensionRegistrar",
+        extensionClass = FirExtensionRegistrarAdapter::class.java
+    )
+}