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 中沒有指定檔案或測試。首先追蹤 nil 檢查警告以及現有的 @nodiscard 註解處理，然後確定應如何表示不返回的內建函式、自訂函式和 @noreturn；當範例不再發出警告且註解行為得到涵蓋時，即表示完成。