Wrap emojis in span nodes (#4759)

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.

test/command/4743.md

@@ -0,0 +1,25 @@
+Test that emojis are wrapped in Span
+
+```
+% pandoc -f commonmark+emoji -t native
+My:thumbsup:emoji:heart:
+^D
+[Para [Str "My",Span ("",["emoji"],[("data-emoji","thumbsup")]) [Str "\128077"],Str "emoji",Span ("",["emoji"],[("data-emoji","heart")]) [Str "\10084\65039"]]]
+```
+
+```
+% pandoc -f markdown+emoji -t native
+My:thumbsup:emoji:heart:
+^D
+[Para [Str "My",Span ("",["emoji"],[("data-emoji","thumbsup")]) [Str "\128077"],Str "emoji",Span ("",["emoji"],[("data-emoji","heart")]) [Str "\10084\65039"]]]
+```
+
+```
+% pandoc -f commonmark+emoji -t html
+:zero: header
+=============
+My:thumbsup:emoji:heart:x :hearts: xyz
+^D
+<h1><span class="emoji" data-emoji="zero">0️⃣</span> header</h1>
+<p>My<span class="emoji" data-emoji="thumbsup">👍</span>emoji<span class="emoji" data-emoji="heart">❤️</span>x <span class="emoji" data-emoji="hearts">♥️</span> xyz</p>
+```

test/command/gfm.md

@@ -38,7 +38,7 @@ gfm tests:
 % pandoc -f gfm -t native
 My:thumbsup:emoji:heart:
 ^D
-[Para [Str "My\128077emoji\10084\65039"]]
+[Para [Str "My",Span ("",["emoji"],[("data-emoji","thumbsup")]) [Str "\128077"],Str "emoji",Span ("",["emoji"],[("data-emoji","heart")]) [Str "\10084\65039"]]]
 ```
 
 ```