add support for column groups in interactive tables

[obsaditelnost]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

At the moment the "columnGroups"-parameter of reactable() is not used for interactive tables (ihtml.active=T)

gt itself supports more than 1 level of column groups but if we have just 1 level of column grouping, it should be possible to add the functionality to the reactable-call

Maybe a warning message if ihtml.active==T & dt_spanners_matrix_height(data,F,F)>1 ?