Detect functions that never return (and @noreturn annotation)

Some functions, such as error, never return control to the caller. This can be used to prove that a value is not nil, for example:

function my_function(input)
  local value = input.value or error('Missing value')
  -- The following line would currently raise a "Needs nil check" warning
  -- However, if it was nil at runtime, an error would have been raised and it would never get this far.
  return value:sub(1, 3)
end

It's not just error that can cause this, however. A custom function could be written that never returns as well (the most obvious way is that the function unconditionally calls error, but there may be other ways involving coroutines or functions implemented in C). A @noreturn annotation could be used to indicate this, similar to @nodiscard.

En el issue no se nombra ningún archivo ni test. Empieza siguiendo la advertencia de comprobación de nil y el manejo existente de la anotación @nodiscard, y determina después cómo deberían representarse los built-ins que no retornan, las funciones personalizadas y @noreturn; se considera terminado cuando el ejemplo ya no emite la advertencia y el comportamiento de la anotación está cubierto.