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

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

@@ -0,0 +1,11 @@
+// EXTRACTION_TARGET: property with initializer
+val a = 1
+
+private val s = "cd"
+
+fun foo(): String {
+    val x = "x$s$a"
+    val y = "${a}${s}x"
+    val z = "xcf$a"
+    return "ab${s}ef"
+}