Refactor xmap_reverse() `weights_into` behaviour to retain unit weights, and only generate new weights when converting fractional weights.

[cynthiahqy]

The current implementation of xmap_reverse() is a bit clunky. it would be better if

• [ ] it retained unit weights in recode maps (only swapping the col_to and col_from) attributes
• [ ] rather than a weights_in argument, it overrides the weights by default (since it will always be adding unit weights).

Once this is implemented:

• [ ] update the 'making-xmaps' vignette

Imagine that instead of converting country codes from ISO Numeric to ISO-2 digit, we wanted to convert from ISO-2 digit to ISO Numeric. We can take the existing crossmap and invert it using unit weights:

```{r}
iso_xmap |>
  xmap_reverse()

We can overwrite the existing weights by using the weights_into argument

iso_xmap |>
  xmap_reverse(weights_into = "link")