Add forest plot

[gravesti]

Depends on #41

• Implement forest plot function from design doc

maicplus/design/Forest_Plot_design.Rmd

Line 336 in 5895afd

## Forest plot function

• Let the function take the output directly from maic_tte_unanchor()

tte_unanchor <- maic_tte_unanchor(useWt, dat, dat_ext, trt, trt_ext, time_scale = "month", endpoint_name = "OS", transform = "log")
maic_forest_plot(tte_unanchor)

• Add test using vdiffr https://vdiffr.r-lib.org/

[hoppanda]

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