feat: add label to refOpts()

[jack-davison]

This PR adds the label and other arguments to refOpts(). It allows for direct labelling of reference lines.

daqi <- openair::importUKAQ(
  year = 2025, 
  source = "aurn", 
  data_type = "daqi", 
  pollutant = "o3"
)

daqi |> 
  dplyr::summarise(
    concentration = max(concentration, na.rm = T), 
    .by = date
  ) |>
  timePlot(
    cols = "black",
    "concentration",
    ref.y = refOpts(
      c(0, 101, 161, 241),
      color = c("#009900FF", "#ff9900FF", "#ff0000FF", "#990099FF"),
      label = c("Low", "Moderate", "High", "Very High")
    )
  )