Case-insensitive comparator for extMimeTypes.

[ThisIsFineTM]

This introduces a case-insensitive lexicographical comparator for the mimetype map in zimwriterfs/tools. This allows us to remove the duplicate entries in the mime type map.

Closes #457.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 28.86%. Comparing base (3ad99a1) to head (1365faa).

Files with missing lines	Patch %	Lines
src/zimwriterfs/tools.cpp	88.88%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
+ Coverage   27.70%   28.86%   +1.16%     
==========================================
  Files          26       26              
  Lines        2534     2477      -57     
  Branches     1363     1297      -66     
==========================================
+ Hits          702      715      +13     
  Misses       1325     1325              
+ Partials      507      437      -70     

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