[JS IR] Use a backing field initializer for js() code folding

During a translation of the code from js() call to statement list,
 the compiler folds the code string. For that it uses property bodies.
 However the bodies can not be loaded in an incremental rebuild.
 The patch uses the backing field initializers of constant properties.
 The initializers are always loaded.

^KT-57002 Fixed

js/js.translator/testData/incremental/invalidation/jsCodeWithConstString/main/m.kt

@@ -0,0 +1,8 @@
+fun box(stepId: Int): String {
+    val x = doTest()
+    when (stepId) {
+        in 0..7 -> if (x != stepId) return "Fail, got $x"
+        else -> return "Unknown"
+    }
+    return "OK"
+}

js/js.translator/testData/incremental/invalidation/jsCodeWithConstString/main/module.info

@@ -0,0 +1,5 @@
+STEP 0:
+    dependencies: lib1
+    added file: m.kt
+STEP 1..7:
+    dependencies: lib1