Add ToC icon display if enableChaptersPlugin

[dualcnhq]

Testing:

• goto: https://deploy-preview-1289--lucid-poitras-9a1249.netlify.app/bookreaderdemo/demo-internetarchive?ocaid=adventureofsherl0000unse
• check if Table Of Contents icon is present in the sidebar menu
• then check other books without ToC like: https://deploy-preview-1289--lucid-poitras-9a1249.netlify.app/bookreaderdemo/demo-internetarchive?ocaid=goody
• the ToC icon should not be present in the page/book without ToC

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a6beb85) 69.28% compared to head (95851b7) 69.29%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1289   +/-   ##
=======================================
  Coverage   69.28%   69.29%           
=======================================
  Files          59       59           
  Lines        5079     5080    +1     
  Branches     1068     1068           
=======================================
+ Hits         3519     3520    +1     
  Misses       1533     1533           
  Partials       27       27           

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[cdrini]

Lgtm ; one suggestion/question.