snippet: Use single quotes in JS
gjs mandates single quotes for strings, which is the closest we get to a canonical "GNOME JS" style, in particular as it is the base for the style guidelines of gnome-shell and friends. As good as it would be to have an option for the quoting style of string literals, it makes more sense to follow the gjs style for now than using double quotes, so adjust the snippets accordingly.
Related: #738