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

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

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