Refactor/move TOC parsing/rendering code

[cdrini]

Part of #8756 . Refactor with no user-discernable effects to move the parsing into table_of_contents.py and create a new helper class to manage tables of contents. Stepping stone for the next piece which will support entry data like authors, etc.

Depends on #9902 .

Technical

Testing

• ✅ books without table of contents render correctly
  • https://testing.openlibrary.org/books/OL10324632M/STOWELL_SYSTEMS_SCIENCE
• ✅ books with "normal" table of contents render correctly
  • https://testing.openlibrary.org/works/OL27482W/The_Hobbit?edition=key%3A/books/OL27289715M
• ✅ books with table of contents + extra fields render correctly
  • https://testing.openlibrary.org/books/OL9462467M/Great_Irish_Stories_of_Childhood
• ✅ books without table of contents can be edited/saved
• ✅ books with "normal" table of contents can be edited/saved
• books with table of contents + extra fields can be edited/saved Punting this to separate PR in order to keep this as a pure refactor.

Screenshot

Stakeholders

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 17.05%. Comparing base (ce16a79) to head (8dd66a9).
Report is 449 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9910      +/-   ##
==========================================
+ Coverage   16.06%   17.05%   +0.99%     
==========================================
  Files          90       89       -1     
  Lines        4769     4725      -44     
  Branches      832      829       -3     
==========================================
+ Hits          766      806      +40     
+ Misses       3480     3409      -71     
+ Partials      523      510      -13     

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[scottbarnes]

Looks good to me.