[fmt_txt] add class and methods

[JanMarvin]

To allow something like this:

library(openxlsx2)

wb <- wb_workbook()$add_worksheet()

txt <- fmt_txt("This Part Bold\n\n ", bold = TRUE) +
  fmt_txt("This part red and underlined!", underline = TRUE, color = wb_color("red"))

c1 <- create_comment(text = txt, author = "")

write_comment(wb, 1, col = "B", row = 10, comment = c1)


wb$add_data(x = txt, colNames = FALSE)
wb$add_cell_style(wrapText = "1")

wb$open()