Unknown column types are silently not written

In this example, `writexl` does not appear to know how to handle a list column, so nothing is written and no warning or error is given. Would it be reasonable to give a warning so people know that their output is not the same as their input data? ``` writexl::write_xlsx(x = tibble(x = list("A", 1)), path = "foo.xlsx") ``` Results in the following file with no data in cells A2 or A3 and no warning.