Use htmltools functions for htmlwidget notebook annotation

[cderv]

This fixes #1762

The issue was in the unpreserving html widget code for UFT8 special character encoded on two byte. (see #1762 (comment)).

Using the htmltools functions to deal with preserved code extraction seems now the way to do it.

I tried to add a test by simplifying the code I used to come up with the solution. Not sure if this is the simplest unit test. Also I added skip_on_cran() because the other test had this too ☺️

In addition, I think this could also have been resolved by making sure to use bytes Encoding before using substrings like in

rmarkdown/R/util.R

Lines 283 to 288 in 1c859f4

	starts_with_bytes <- function(string, bytes) {
	Encoding(string) <- Encoding(bytes) <- "bytes"
	if (n_bytes(bytes) > n_bytes(string))
	return(FALSE)
	substring(string, 1, n_bytes(bytes)) == bytes
	}

However, htmltools functions seemed dedicated to this usage.

[yihui]

It makes perfect sense to use htmltools. Thanks!