Extract Function/Introduce Property: Support extraction of string template fragments

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

@@ -0,0 +1,9 @@
+// EXTRACTION_TARGET: property with initializer
+val a = 1
+
+fun foo(): String {
+    val x = "xcd$a"
+    val y = "${a}cdx"
+    val z = "xcf$a"
+    return "ab<selection>cd</selection>ef"
+}