Switch Observable to be a non-marker protocol

With support for redundant conformance declarations via macros,
the `Observable` protocol can be a non-marker protocol, which
provides more flexibility for evolution in the future.

rdar://111463883

Author: natecook1000

stdlib/public/Observation/Sources/Observation/Observable.swift

@@ -18,7 +18,7 @@
 /// the ``Observation/Observable-swift.macro`` macro when adding observation
 /// support to a type.
 @available(SwiftStdlib 5.9, *)
-@_marker public protocol Observable { }
+public protocol Observable { }
 
 #if $Macros && hasAttribute(attached)