`simplify_image_links()` fails to simplify because of unexpected html comment strings

[mrcaseb]

The function that is supposed to simplify the image strings expects strings of the form ![](https://i.imgur.com/woc4vHs.png).

reprex/R/reprex_render.R

Lines 335 to 339 in 07cd5d7

	# input: 
	# output: https://i.imgur.com/woc4vHs.png
	simplify_image_links <- function(x) {
	sub("(^!\\[\\]\\()(.+)(\\)$)", "\\2", x, perl = TRUE)
	}

If this was the input, the code would work fine

x <- "![](https://i.imgur.com/woc4vHs.png)"
sub("(^!\\[\\]\\()(.+)(\\)$)", "\\2", x, perl = TRUE)
#> [1] "https://i.imgur.com/woc4vHs.png"

However, the actual input includes empty html strings and has the form ![](https://i.imgur.com/woc4vHs.png)<!-- -->

In this case, the function fails to simplify the link

x <- "![](https://i.imgur.com/woc4vHs.png)<!-- -->"
sub("(^!\\[\\]\\()(.+)(\\)$)", "\\2", x, perl = TRUE)
#> [1] "![](https://i.imgur.com/woc4vHs.png)<!-- -->"

The result is that links aren't simplified when doing reprex(venue = "slack") as the below example shows (I put the image link in backticks)

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

![](https://i.imgur.com/9hBJEWw.png)<!-- -->