deprecate unsafe host-specific bindings from stdlib

Fixes rescript-lang/rescript-core#240

Author: tsnobip

runtime/Stdlib.res

@@ -53,8 +53,11 @@ type null<+'a> = Primitive_js_extern.null<'a>
 type undefined<+'a> = Primitive_js_extern.undefined<'a>
 type nullable<+'a> = Primitive_js_extern.nullable<'a>
 
+@deprecated("Use rescript-webapi instead")
 @val external window: Dom.window = "window"
+@deprecated("Use rescript-webapi instead")
 @val external document: Dom.document = "document"
+@deprecated("Use rescript-webapi instead")
 @val external globalThis: {..} = "globalThis"
 
 /**