Add type annotation to main in projects generated by gleam new

[drewolson]

Currently, the gleam new command generates a project with a main function without a type annotation. It is idiomatic in gleam to include type annotations, and therefore I'd suggest we add an annotation to main of the type -> Nil.

[lpil]

I like that this will encourage people to annotate their functions, and maybe it'll persuade people not to try and return a result from their main function. It might be a slight annoyance if you do want to change the return type, but I'm not sure why you would want to return some other type.

I'm in favour of this, but I'm open to any other suggestions!

[drewolson]

I've created a PR that resolves this issue. Once everyone agrees it is a reasonable approach, we can merge it.