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

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

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