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.

Issue ではファイルもテストも指定されていません。まず nil チェックの警告と既存の @nodiscard アノテーション処理を追跡し、次に戻り値を返さない built-in、カスタム関数、@noreturn をどのように表現すべきかを判断してください。例で警告が出なくなり、アノテーションの動作がカバーされていれば完了です。