nested groups

[r2evans]

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Prework

• [x] Read and abide by the great_tables code of conduct and contributing guidelines.
• [x] Search for duplicates among the existing issues (both open and closed).

Proposal

Implement "nested groups" as originally requested in https://github.com/rstudio/gt/issues/399 and backport to gt.

Thanks!

[deanm0000]

Is this merging cells? If so, big fan of this idea.

[RegalPlatypus]

Commenting to add that I would really love to see indented nesting of multiple group_by() variables.