Add seamless integration with gtsummary, marginaleffects, emmeans, etc.

[thatoneguy006]

We can speed up the forest plot creation process by adding integration with gtsummary and other modeling packages.

[thatoneguy006]

gtsummary:
as_forest_data.tbl_regression()
as_forest_data.tbl_uvregression()
as_forest_data.tbl_merge()
as_forest_data.tbl_stack()

marginaleffects:
as_forest_data.comparisons()
as_forest_data.avg_comparisons()
as_forest_data.predictions()

emmeans:
as_forest_data.emmGrid()
as_forest_data.summary_emm()
as_forest_data.pairs_emm()

[p-lemercier]

Hi,
I'm very interested in this enhancement. As for gtsummary, it might be worth checking out this project: https://github.com/ddsjoberg/gtforester/

Thank you for your amazing work