Move syntax hilight language name to data-* attribute

The current behavior is to get the language name from the text content of the node.
I believe it too fragile, because we can't add languages with more than 1 name (Visual Basic for instance) or with special characters (C# for instance).

My proposal is to transfer the "syntax language name" to a data-* attribute.

I've also renamed all the current supported languages to their real name (with capital letters where applicable).