Add font_size argument to gt_plt_sparkline()

[rossellhayes]

This PR adds a font_size argument to gt_plt_sparkline() to allow setting a custom font size for numeric labels. Closes #154.

mtcars %>%
    dplyr::group_by(cyl) %>%
    dplyr::summarize(mpg_data = list(mpg), .groups = "drop") %>%
    gt() %>%
    gt_plt_sparkline(mpg_data, font_size = 2)

mtcars %>%
    dplyr::group_by(cyl) %>%
    dplyr::summarize(mpg_data = list(mpg), .groups = "drop") %>%
    gt() %>%
    gt_plt_sparkline(mpg_data, font_size = 5)