Handle Ast::Discard when building the scope tree.

Author: gdotdesign

spec/examples/discard

@@ -0,0 +1,8 @@
+component Main {
+  fun render : String {
+    case "A" {
+      "A" => "A"
+      _ => "X"
+    }
+  }
+}

src/scope.cr

@@ -218,6 +218,7 @@ module Mint
            Ast::Variable,
            Ast::Builtin,
            Ast::Comment,
+           Ast::Discard,
            Ast::Env
       when Ast::StringLiteral,
            Ast::HereDocument,