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.

1 files changed, 156 insertions, 0 deletions

diff --git a/test/command/4528.md b/test/command/4528.md
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+# Rendering small caps, superscripts and subscripts with and without `raw_html`
+
+## Small caps
+
+```
+% pandoc --wrap=none -f latex -t commonmark-raw_html
+This has \textsc{small caps} in it.
+^D
+This has SMALL CAPS in it.
+```
+
+```
+% pandoc --wrap=none -f latex -t commonmark+raw_html
+This has \textsc{small caps} in it.
+^D
+This has <span class="smallcaps">small caps</span> in it.
+```
+```
+
+```
+% pandoc --wrap=none -f latex -t markdown_strict+raw_html
+This has \textsc{small caps} in it.
+^D
+This has <span class="smallcaps">small caps</span> in it.
+```
+
+## Strikeout
+
+```
+% pandoc --wrap=none -f html -t commonmark-raw_html-strikeout
+This has <s>strikeout</s> in it.
+^D
+This has strikeout in it.
+
+```
+% pandoc --wrap=none -f html -t commonmark+raw_html-strikeout
+This has <s>strikeout</s> in it.
+^D
+This has <s>strikeout</s> in it.
+```
+
+```
+% pandoc --wrap=none -f html -t commonmark-raw_html+strikeout
+This has <s>strikeout</s> in it.
+^D
+This has ~~strikeout~~ in it.
+```
+
+```
+% pandoc --wrap=none -f html -t commonmark+raw_html+strikeout
+This has <s>strikeout</s> in it.
+^D
+This has ~~strikeout~~ in it.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict-raw_html-strikeout
+This has <s>strikeout</s> in it.
+^D
+This has strikeout in it.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html-strikeout
+This has <s>strikeout</s> in it.
+^D
+This has <s>strikeout</s> in it.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict-raw_html+strikeout
+This has <s>strikeout</s> in it.
+^D
+This has ~~strikeout~~ in it.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html+strikeout
+This has <s>strikeout</s> in it.
+^D
+This has ~~strikeout~~ in it.
+```
+
+## Superscript
+
+```
+% pandoc --wrap=none -f html -t commonmark-raw_html
+This has <sup>superscript</sup> in it and <sup>2 3</sup> again. With emphasis: <sup><em>2</em> 3</sup>. With letters: <sup>foo</sup>. With a span: <sup><span class=foo>2</span></sup>.
+^D
+This has ^(superscript) in it and ² ³ again. With emphasis: ^(*2* 3). With letters: ^(foo). With a span: ².
+```
+
+```
+% pandoc --wrap=none -f html -t commonmark+raw_html
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+^D
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict-raw_html-superscript
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+^D
+This has ^(superscript) in it and ² again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html-superscript
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+^D
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html+superscript
+This has <sup>superscript</sup> in it and <sup>2</sup> again.
+^D
+This has ^superscript^ in it and ^2^ again.
+```
+
+## Subscript
+
+```
+% pandoc --wrap=none -f html -t commonmark-raw_html
+This has <sub>subscript</sub> in it and <sub>2 3</sub> again. With emphasis: <sub><em>2</em> 3</sub>. With letters: <sub>foo</sub>. With a span: <sub><span class=foo>2</span></sub>.
+^D
+This has \_(subscript) in it and ₂ ₃ again. With emphasis: \_(*2* 3). With letters: \_(foo). With a span: ₂.
+```
+
+```
+% pandoc --wrap=none -f html -t commonmark+raw_html
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+^D
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict-raw_html-subscript
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+^D
+This has _(subscript) in it and ₂ again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html-subscript
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+^D
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+```
+
+```
+% pandoc --wrap=none -f html -t markdown_strict+raw_html+subscript
+This has <sub>subscript</sub> in it and <sub>2</sub> again.
+^D
+This has ~subscript~ in it and ~2~ again.
+```