[styles] allow updates of font elements

[JanMarvin]

Related to this SO. If update = TRUE all non empty font elements will be overwritten. At the moment it is not possible to set a font element to missing. All elements are initialized with either "" or some value. Might need some additional thinking

library(openxlsx2)

wb <- wb_workbook() |>
  wb_add_worksheet() |>
  wb_add_data(x = letters) |>
  wb_add_font(dims = wb_dims(x = letters), name = "Calibri", size = 20) |>
  # updates only the font color
  wb_add_font(dims = wb_dims(x = letters), name = "", size = "", color = wb_color("orange"), update = TRUE)

if (interactive()) wb$open()