ENH: `Styler.apply(map)_index` made compatible with `Styler.to_latex`
This is a follow-on to #41893.

It makes the new `Styler.apply_index` and `Styler.applymap_index` methods compatible with `Styler.to_latex`.

![Screen Shot 2021-06-14 at 09 24 46](https://user-images.githubusercontent.com/24256554/121854567-a22de200-ccf2-11eb-820a-cc905c7aac3a.png)


