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

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

@@ -0,0 +1,9 @@
+// SUGGESTED_NAMES: s, getX
+fun foo(a: Int): String {
+    val x = "x${s()}$a"
+    val y = "${a}${s()}x"
+    val z = "xcf$a"
+    return "ab${s()}ef"
+}
+
+private fun s() = "cd"