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

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

@@ -0,0 +1,10 @@
+// SUGGESTED_NAMES: i, getX
+fun foo(param: Int): String {
+    val x = "a${i()}4_"
+    val y = "-4${i()}a"
+    val z = "+1243a"
+    val u = 123
+    return "ab${i()}def"
+}
+
+private fun i() = 123