improve fillMergedCells. closes #324

[JanMarvin]

Now the following is possible

library(openxlsx2)

# Create a workbook where the second row are merged cells from the first row
wb <- wb_workbook()$
  add_worksheet()$
  add_data(x = t(letters[1:4]), colNames = FALSE)$
  add_data(dims = "A3:D6", x = t(matrix(c(1:3, NA_real_), 4, 4)), colNames = FALSE)$
  merge_cells(1, rows = 1:2, cols = 1)$
  merge_cells(1, rows = 1:2, cols = 2)$
  merge_cells(1, rows = 1:2, cols = 3)$
  merge_cells(1, rows = 1:2, cols = 4)

# read the workbook from the second row and fill the merged 
# cells, even though they are outside the selected dimension
wb_to_df(wb, dims = "A2:D4", fillMergedCells = TRUE)
#>   a b c  d
#> 3 1 2 3 NA
#> 4 1 2 3 NA

Presumably with large xlsxFiles it is faster to read from a wbWorkbook instead of reading from a file.