cask: add DSL methods to RBI file

Library/Homebrew/cask/cask.rbi

@@ -2,10 +2,44 @@
 
 module Cask
   class Cask
+    def appcast; end
+
     def artifacts; end
 
+    def auto_updates; end
+
+    def caveats; end
+
+    def conflicts_with; end
+
+    def container; end
+
+    def desc; end
+
+    def depends_on; end
+
     def homepage; end
 
+    def language; end
+
+    def languages; end
+
+    def name; end
+
+    def sha256; end
+
     def staged_path; end
+
+    def url; end
+
+    def version; end
+
+    def appdir; end
+
+    def discontinued?; end
+
+    def livecheck; end
+
+    def livecheckable?; end
   end
 end