vec_is_list() tests if
x is considered a list in the vctrs sense. It
x is a bare list with no class.
x is a list explicitly inheriting from
Notably, data frames and S3 record style classes like POSIXlt are not considered lists.
x inherits explicitly from
"list", it is also required that the
proxy returned by
vec_proxy() is a list. If it is not, an error is thrown.
#>  TRUE#>  TRUE#>  FALSE