Add `conflict_prefer` for `read_xls`

```
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.

Start by reproducing the reported conflict with inventory, readxl, and tbltools, then inspect how the package currently handles conflicted functions. Confirm which preference the project intends for read_xls; done means the package declares that preference and the shown conflict no longer occurs.

Written by the indexing model from the issue text.