add warning that alerts user when they inadvertently attempt to remove index from tsibble?

[david-romano]

Before:

library(tidyverse)
library(tsibble)

tsibble(
  a = parse_date('1/1/24', format = '%m/%d/%y'),
  b = 1,
)
#> Using `a` as index variable.
#> # A tsibble: 1 x 2 [?]
#>   a              b
#>   <date>     <dbl>
#> 1 2024-01-01     1

After select(-a):

tsibble(
  a = parse_date('1/1/24', format = '%m/%d/%y'),
  b = 1,
) |> 
  select(-a)
#> Using `a` as index variable.
#> # A tsibble: 1 x 2 [?]
#>       b a         
#>   <dbl> <date>    
#> 1     1 2024-01-01

^{Created on 2024-03-18 with reprex v2.0.2}

(See discussion at Behavior of select when trying to remove the index from a tsibble.)