If *isic* is numeric we should throw a warning or error

[maurolepore]

Relates to https://github.com/2DegreesInvesting/tiltIndicatorBefore/pull/69
Relates to 2DegreesInvesting/tiltIndicator#636

Now we expect single quotes around values of *isic*. This should nudge all reading functions to interpret *isic* as a character and protect leading zeroes.

If the user passes a dataset where *isic* is numeric, then we should throw a warning like this:

rlang::warn(c(
  "`*isic_4digit` should be a character but it's numeric.",
  i = "Are you using outdated data?"
))
#> Warning: `*isic_4digit` should be a character but it's numeric.
#> ℹ Are you using outdated data?

^{Created on 2023-12-06 with reprex v2.0.2}