expand gt_group to handle both gt_groups and gt_tbls

[shannonhaughton]

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

It would be nice if given a list of gt_tbl and gt_group objects, you could call gt_group and it would collapse them all into a gt_group object. Currently this only works for lists of gt_tbls, but in our work using other packages such as tfrmt we combine gt_tbls and gt_groups together

[rich-iannone]

Thank you for this issue and the associated PR! Closing this issue since #2129 is now merged.