Text.Pandoc.Emoji now exports `emojiToInline`, which returns a Span inline containing the emoji character and some attributes with metadata (class `emoji`, attribute `data-emoji` with emoji name). Previously, emojis (as supported in Markdown and CommonMark readers, e.g "😄")
were simply translated into the corresponding unicode code point. By wrapping them in Span
nodes, we make it possible to do special handling such as giving them a special font
in HTML output. We also open up the possibility of treating them differently when the
`--ascii` option is selected (though that is not part of this commit).
Closes#4743.
