Add `@sbtUnchecked` to sbt 0.13.16

Fixes #3337

main/settings/src/main/scala/sbt/sbtUnchecked.scala

@@ -0,0 +1,13 @@
+package sbt
+
+import scala.annotation.Annotation
+
+/** An annotation to designate that the annotated entity
+ *  should not be considered for additional sbt compiler checks.
+ *  These checks ensure that the DSL is predictable and prevents
+ *  users from doing dangerous things at the cost of a stricter
+ *  code structure.
+ *
+ *  @since 0.13.16
+ */
+class sbtUnchecked extends Annotation