This makes two improvements to main.

1. be honest regarding the return type

In C++ main has a special rule allowing it to not return the promised
value. Instead of allowing this artefact in cpp2, require that main
returns the promised value. When main returns no value, let the user
specify that by returning void.

2. make the arguments cpp2 compatible

The argv of main is not compatible with cpp2. Instead of making this a
special case in cpp2, use different parameter for main.
A std::span<std::string_view> is a better safe alternative for argv so
use that instead.
Note the implementation stores the argv in a std::vector, but it seems
better to use a std::span in the interface. If, in the future, a more
suitable container is added to C++ this doesn't change the interface in
the new main function.
It would be possible to use different parameters types like:
- std::vector<std::string_view>
- std::vector<std::string>
- std::span<std::string>
or even allow all 4 of them. I decided against multiple types to keep
things simple to teach and not let the number of options grow when more
suitable containers are added to the Standard, for example
   P0843 static_vector

Both improvements are implemented by adding a new implementation defined
function. This function cpp2__main replaces the name main and keeps the
body if the function. Then it generates a helper main function to call
cpp__main with the appropriate arguments.

This removes teaching the special exception for main in [stmt.return]/4
  Otherwise, flowing off the end of a function that is neither main
  ([basic.start.main]) nor a coroutine ([dcl.fct.def.coroutine]) results in
  undefined behavior.