kotlinx.atomicfu compiler plugin for JS_IR backend (#4581)

* kotlinx.atomicfu compiler plugin for JS_IR

Support transformations of atomic operations introduced by the kotlinx.atomicfu library for the JS_IR backend. Compiler plugin is applied externally by the kotlinx.atomicfu gradle plugin.

* Apply compiler plugin for JS platform only

* New plugin test structure

* testGroupOutputDirPrefix changed

plugins/atomicfu/atomicfu-compiler/testData/box/SynchronizedObjectTest.kt

@@ -0,0 +1,21 @@
+import kotlinx.atomicfu.locks.*
+import kotlin.test.*
+
+class SynchronizedObjectTest : SynchronizedObject() {
+
+    fun testSync() {
+        val result = synchronized(this) { bar() }
+        assertEquals("OK", result)
+    }
+
+    private fun bar(): String =
+        synchronized(this) {
+            "OK"
+        }
+}
+
+fun box(): String {
+    val testClass = SynchronizedObjectTest()
+    testClass.testSync()
+    return "OK"
+}