Add Task as a built-in module/type

crates/compiler/uitest/tests/recursive_type/fix_recursion_under_alias_issue_4368.txt

@@ -4,10 +4,10 @@ Effect : [
     DoIt {} ({} -> Effect),
 ]
 
-Task := ({} -> Effect) -> Effect
+MockTask := ({} -> Effect) -> Effect
 
-doIt : {} -> Task
+doIt : {} -> MockTask
 doIt = \{} ->
-#^^^^{-1} {} -[[doIt(0)]]-> Task
-    @Task \toNext ->
+#^^^^{-1} {} -[[doIt(0)]]-> MockTask
+    @MockTask \toNext ->
         DoIt {} \{} -> (toNext {})

crates/compiler/uitest/tests/solve/opaque_and_alias_unify.txt

@@ -2,8 +2,8 @@ app "test" provides [always] to "./platform"
 
 Effect a := {} -> a
 
-Task a err : Effect (Result a err)
+MockTask a err : Effect (Result a err)
 
-always : a -> Task a *
+always : a -> MockTask a *
 always = \x -> @Effect (\{} -> Ok x)
-#^^^^^^{-1} a -[[always(0)]]-> Task a *
+#^^^^^^{-1} a -[[always(0)]]-> MockTask a *

crates/compiler/uitest/tests/solve/task_wildcard_wildcard.txt

@@ -1,11 +1,7 @@
 app "test" provides [tforever] to "./platform"
 
-Effect a := {} -> a
+Task ok err := {} -> ok err
 
-eforever : Effect a -> Effect b
-
-Task a err : Effect (Result a err)
-
-tforever : Task val err -> Task * *
-tforever = \task -> eforever task
+tforever : Task val * -> Task * *
+tforever = \task -> Task.forever task
 #^^^^^^^^{-1} Task val err -[[tforever(0)]]-> Task * *