Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

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

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