Add option to fetch multiple textsel pages in one request

[cdrini]

New TextSelectionPlugin option, fetchBatchSize, can be used to fetch multiple djvu xml pages in one request. The endpoint must support pageIndex being a comma-separated list.

[codecov]

Codecov Report

Attention: Patch coverage is 22.38806% with 52 lines in your changes missing coverage. Please review.

Project coverage is 68.64%. Comparing base (fc3ebf7) to head (8029363).

Files with missing lines	Patch %	Lines
src/util/debouncer.js	24.39%	31 Missing ⚠️
src/plugins/plugin.text_selection.js	19.23%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1388      +/-   ##
==========================================
- Coverage   69.24%   68.64%   -0.60%     
==========================================
  Files          60       61       +1     
  Lines        5166     5231      +65     
  Branches     1111     1119       +8     
==========================================
+ Hits         3577     3591      +14     
- Misses       1558     1609      +51     
  Partials       31       31              

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