Add the fmt_chem() function

[rich-iannone]

With fmt_chem() it's possible to format chemical formulas and chemical equations. The markup necessary for this is called gt's chemistry notation. One example of fmt_chem() usage is in this example:

reactions |>
  dplyr::filter(cmpd_type == "terminal monoalkene") |>
  dplyr::filter(grepl("^1-", cmpd_name)) |>
  dplyr::select(cmpd_name, cmpd_formula, ends_with("k298")) |>
  gt() |>
  sub_missing() |>
  fmt_chem(columns = cmpd_formula) |>
  fmt_scientific() |>
  cols_label(
    cmpd_name = "Alkene",
    cmpd_formula = "Formula",
    oh_k298 = "OH",
    o3_k298 = "{{%O3%}}",
    no3_k298 = "{{%NO3%}}",
    cl_k298 = "Cl"
  ) |>
  tab_spanner(
    label = "Reaction Rate Constant at 298 K",
    columns = ends_with("k298")
  ) |>
  tab_header(title = "Gas-phase reactions of selected terminal alkenes") |>
  opt_align_table_header(align = "left")