Missing info about prelude / built-in types and Nil

[inoas]

• Result is built in verus Option and Dict are not
• the option and dict examples in the do not import the type but it is required if people reference it in their signatures and newbies stumble, the exercism has it https://exercism.org/tracks/gleam/concepts (buts the Dict concept could use a type example over there)
• if the tour grts to have a prelude chapter, maybe the exercism concepts can have almost the same?