CommonMark writer: add plain text fallbacks. (#4531)

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.

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