Surround with braces when completing backing field in string template

idea/testData/completion/handlers/basic/stringTemplate/BackingField.kt

@@ -0,0 +1,10 @@
+class C {
+    var property = "abc"
+        get() = $property + 1
+
+    fun foo(){
+        "$<caret>"
+    }
+}
+
+// ELEMENT: $property

idea/testData/completion/handlers/basic/stringTemplate/BackingField.kt.after

@@ -0,0 +1,10 @@
+class C {
+    var property = "abc"
+        get() = $property + 1
+
+    fun foo(){
+        "${$property<caret>}"
+    }
+}
+
+// ELEMENT: $property