Extraction Engine: Generate function/property accessor with expression body (whenever applicable)

#KT-6405 Fixed

idea/testData/refactoring/extractFunction/initializers/functions/topLevelFunction.kt.after

@@ -4,6 +4,4 @@
 // PARAM_DESCRIPTOR: value-parameter val b: kotlin.Int defined in foo
 fun foo(a: Int, b: Int) = i(a, b) - 1
 
-private fun i(a: Int, b: Int): Int {
-    return a + b
-}
+private fun i(a: Int, b: Int) = a + b