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/SimpleLockTest.kt

@@ -0,0 +1,36 @@
+import kotlinx.atomicfu.*
+import kotlin.test.*
+
+class SimpleLockTest {
+    fun withLock() {
+        val lock = SimpleLock()
+        val result = lock.withLock {
+            "OK"
+        }
+        assertEquals("OK", result)
+    }
+}
+
+class SimpleLock {
+    private val _locked = atomic(0)
+
+    fun <T> withLock(block: () -> T): T {
+        // this contrieves construct triggers Kotlin compiler to reuse local variable slot #2 for
+        // the exception in `finally` clause
+        try {
+            _locked.loop { locked ->
+                check(locked == 0)
+                if (!_locked.compareAndSet(0, 1)) return@loop // continue
+                return block()
+            }
+        } finally {
+            _locked.value = 0
+        }
+    }
+}
+
+fun box(): String {
+    val testClass = SimpleLockTest()
+    testClass.withLock()
+    return "OK"
+}