Implement venue = "discord"

[mrcaseb]

This implements #402 and while working on it I realized that I can fix #473

This PR

• allows the "discord" venue in various match.arg calls
• adds pp_discordify() which is almost exactly the same as pp_slackify(), except that it removes a blank space after the opening backticks to trigger discord syntax highlighting (see Discord Venue with Code Highlighting #402).
• fixes a problem in simplify_image_links() which is called by both pp_discordify() and pp_slackify() (see simplify_image_links() fails to simplify because of unexpected html comment strings #473)
• adds a test for venue = "discord"

[mrcaseb]

Here is a reprex of the discord venue

reprex::reprex_discord(
  x = {
    library(ggplot2)
    ggplot(mtcars, aes(disp, qsec)) +
      geom_point()
  }
)

Output:

library(ggplot2)
ggplot(mtcars, aes(disp, qsec)) +
  geom_point()

https://i.imgur.com/LEndv9l.png