[write_data] escape all formulas in write_data

[JanMarvin]

This solves an issue described in this SO thread. Previously data frame formulas were not xml escaped.

library(openxlsx2)

df_tmp <- data.frame(
  f = paste0("'A&B'!A1")
)
class(df_tmp$f) <- c(class(df_tmp$f), "formula")

wb <- wb_workbook()$
  add_worksheet("A&B")$
  add_worksheet("Fml")$add_data(x = df_tmp, col_names = FALSE)

if (interactive()) wb$open()