Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

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

@@ -0,0 +1,7 @@
+fun foo(a: Int): String {
+    val i = a + 1
+    val x = "-$i"
+    val y = "x${i}y"
+    val z = "x${a - 1}y"
+    return "abc${i}def"
+}