JS: add boxing/unboxing to default accessors of non-simple properties

See KT-21421

js/js.translator/testData/box/coercion/defaultAccessors.kt

@@ -0,0 +1,29 @@
+// EXPECTED_REACHABLE_NODES: 1140
+interface I {
+    val a: Char
+}
+
+object X : I {
+    override var a = '#'
+}
+
+var result = ""
+
+object Y : I {
+    override var a = '#'
+        get() {
+            result = jsTypeOf(field.asDynamic())
+            return field
+        }
+}
+
+fun box(): String {
+    val t = jsTypeOf(X.asDynamic().a)
+    if (t != "object") return "fail1: $t"
+
+    Y.a = '@'
+    Y.a
+    if (result != "number") return "fail2: $result"
+
+    return "OK"
+}