Flexible merge

[JanMarvin]

This PR provides a solver for intersecting cell regions

library(openxlsx2)
wb <- wb_workbook()$add_worksheet()
wb$merge_cells(dims = "A1:B2")
wb$merge_cells(dims = "G1:K2")
wb$merge_cells(dims = "A1:C3", solve = TRUE)
wb$merge_cells(dims = "A1:J7", solve = TRUE)
wb$merge_cells(dims = "A1:Z8", solve = TRUE)
wb$merge_cells(dims = "B2:D4", solve = TRUE)
if (interactive) wb$open()