The syntax script allowed for single backslash escapes like this
"string\""
But did not accommodate for the uncommon case:
"<key>\\"
Let's fix this by also skipping over double backslashes in the
skillString region.
closes: vim/vim#15832cb1d1dcc87
Co-authored-by: Simão Afonso @ Powertools Tech <simao.afonso@powertools-tech.com>