[FIX] io_util: Detect utf-8-sig when using file utility

[ales-erjavec]

Issue

Detect utf-8-sig when using file utility.

Fixes #7000

Note: An alternative would be to remove the use of file utility, chardet would recognize the BOM (and does so on windows where file is typically not available).

Description of changes

Detect utf-8-sig when using file utility.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.48%. Comparing base (091e88c) to head (2df43bf).
Report is 65 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7006   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files         332      332           
  Lines       73333    73341    +8     
=======================================
+ Hits        64889    64897    +8     
  Misses       8444     8444           

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