Previously, the writer would unconditionally emit HTMLish output for
subscripts, superscripts, strikeouts (if the strikeout extension is
disabled) and small caps, even with raw_html disabled.
Now there are plain-text (and, where possible, fancy Unicode)
fallbacks for all of these corresponding (mostly) to the Markdown
fallbacks, and the HTMLish output is only used when raw_html is
enabled.
This commit adds exported functions `toSuperscript` and
`toSubscript` to `Text.Pandoc.Writers.Shared`. [API change]
Closes#4528.
