Improve warning message when passing a character vector to .before/.after

[arnaudgallou]

Here's the warning raised by the following code:

var <- "y"
tibble(y = seq(letters)) |> mutate(x = letters, .before = var)

# Using an external vector in selections was deprecated in tidyselect 1.1.0.
# ℹ Please use `all_of()` or `any_of()` instead.
#   # Was:
#   data %>% select(var)
#   
#   # Now:
#   data %>% select(all_of(var))
# See <https://tidyselect.r-lib.org/reference/faq-external-vector.html>.
# This warning is displayed once every 8 hours.
# Call `lifecycle::last_lifecycle_warnings()` to see where this warning was generated. 

I find this warning message misleading and unclear. In a script using both select() and mutate(), the message distracts us from the actual function raising the warning. In a script with no select(), it's unclear the warning originate from mutate's .before/.after.

[DavisVaughan]

Tracking in r-lib/tidyselect#334