Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

idea/testData/refactoring/introduceVariable/stringTemplates/singleEntrySubstring.kt.after

@@ -0,0 +1,7 @@
+fun foo(a: Int): String {
+    val s = "cd"
+    val x = "x$s$a"
+    val y = "${a}${s}x"
+    val z = "xcf$a"
+    return "ab${s}ef"
+}