Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

idea/testData/refactoring/introduceVariable/stringTemplates/fullEntryWithSimpleName.kt

@@ -0,0 +1,6 @@
+fun foo(a: Int): String {
+    val x = "-$a"
+    val y = "x${a}y"
+    val z = "x$ay"
+    return "abc<selection>${a}</selection>def"
+}