Support TargetPlatform.isJs() for FIR

core/compiler.common/src/org/jetbrains/kotlin/platform/JsPlatform.kt

@@ -0,0 +1,15 @@
+/*
+ * Copyright 2010-2022 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.platform
+
+abstract class JsPlatform : SimplePlatform("JS") {
+    override val oldFashionedDescription: String
+        get() = "JavaScript "
+}
+
+// TODO: temporarily conservative implementation; use the same approach as for TargetPlatform?.isNative()
+//  when JsPlatform will become parameterized with "JS target"
+fun TargetPlatform?.isJs(): Boolean = this?.singleOrNull() is JsPlatform