Migrate TtsPlugin to BookReaderPlugin

[cdrini]

Blocked by #1374 , so ignore the autoplay files in this diff

Breaking Changes:

• All root-level BookReader tts methods/fields, eg BookReader#ttsToggle, BookReader#ttsScrollToChunk, BookReader#ttsStart, etc, have all been moved to e.g. br._plugins.tts.toggle. That is they are now on the TtsPlugin class, and no longer have the tts as in ttsStart.
• The option enableTtsPlugin has been renamed to plugins.tts.enabled
• The options server and bookPath have been replaced with pageChunkUrl StringWithVars. This makes the page chunk URL configurable for other clients.

[codecov]

Codecov Report

Attention: Patch coverage is 68.35443% with 25 lines in your changes missing coverage. Please review.

Project coverage is 69.22%. Comparing base (c067962) to head (9a2e689).

Files with missing lines	Patch %	Lines
src/plugins/plugin.autoplay.js	70.83%	14 Missing ⚠️
src/BookReader.js	70.00%	5 Missing and 1 partial ⚠️
src/BookReader/Mode2UpLit.js	0.00%	2 Missing ⚠️
src/plugins/tts/PageChunk.js	0.00%	1 Missing ⚠️
src/plugins/tts/PageChunkIterator.js	0.00%	1 Missing ⚠️
src/plugins/url/plugin.url.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1376      +/-   ##
==========================================
- Coverage   69.25%   69.22%   -0.03%     
==========================================
  Files          60       60              
  Lines        5135     5150      +15     
  Branches     1087     1102      +15     
==========================================
+ Hits         3556     3565       +9     
- Misses       1550     1555       +5     
- Partials       29       30       +1     

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