Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

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

@@ -0,0 +1,6 @@
+fun foo(a: Int): String {
+    val x = "xcd$a"
+    val y = "${a}cdx"
+    val z = "xcf$a"
+    return "ab<selection>cd</selection>ef"
+}