Split ReplaceSingleLineLetInspection to SimpleRedundantLet & ComplexRedundantLet

Relates to #KT-32010

idea/testData/inspectionsLocal/complexRedundantLet/simple.kt

@@ -0,0 +1,8 @@
+// PROBLEM: none
+// WITH_RUNTIME
+
+fun test(s: String?): Int? {
+    return s?.let<caret> {
+        it.length
+    }
+}