Add `conflict_prefer` for `read_xls`

[arilamstein]

library(inventory)
library(readxl)
read_xls("asdf")
Error: [conflicted] `read_xls` found in 2 packages.
Either pick the one you want with `::` 
* readxl::read_xls
* tbltools::read_xls
Or declare a preference with `conflict_prefer()`
* conflict_prefer("read_xls", "readxl")
* conflict_prefer("read_xls", "tbltools")

I think that readxl is not loaded when you type library(tidyverse) which is why we missed this before.