add_comment() references the "CommentReference" character style without defining it

[Dmitry-Kov]

Document.add_comment() (via Run.mark_comment_range) writes the comment reference run with rStyle w:val="CommentReference", but nothing adds that style to styles.xml. A default Document() template doesn't contain it either. Word and LibreOffice both tolerate a dangling rStyle silently and format the reference mark with the run's default properties instead of the built-in 8pt "annotation reference" style, so the mark just looks slightly wrong with no warning anywhere.

Repro, python-docx 1.2.0:

from docx import Document
import zipfile

d = Document()
p = d.add_paragraph("The fee is EUR 12,000 per milestone.")
d.add_comment(runs=p.runs[0], text="Confirm this figure", author="Reviewer")
d.save("out.docx")

z = zipfile.ZipFile("out.docx")
doc = z.read("word/document.xml").decode()
sty = z.read("word/styles.xml").decode()
print('rStyle in document.xml:', doc.count('w:val="CommentReference"'))
print('style defined in styles.xml:', sty.count('w:styleId="CommentReference"'))

# rStyle in document.xml: 1
# style defined in styles.xml: 0

Suggested fix: when adding a comment, add the style definition if the document lacks it. What Word writes is:

<w:style w:type="character" w:styleId="CommentReference">
  <w:name w:val="annotation reference"/>
  <w:basedOn w:val="DefaultParagraphFont"/>
  <w:uiPriority w:val="99"/>
  <w:semiHidden/><w:unhideWhenUsed/>
  <w:rPr><w:sz w:val="16"/><w:szCs w:val="16"/></w:rPr>
</w:style>

Alternatively, omit the rStyle altogether when the style isn't defined — a reference mark with default formatting is at least self-consistent.

Filing this separately per the discussion in #1605. The same gap existed in adeu and was fixed in their 3.0.3.

python-docx 1.2.0, Python 3.9.6, macOS