[ENH] Snowball - Use ISO language codes

[PrimozGodec]

Issue

This PR is part of #963, which I am splitting into smaller pieces for easier review.

The main motivation behind this is to make Preprocess work with language from Corpus.

Description of changes

This PR prepare a Snowball normalizer to communicate (get and return languages) as ISO codes, which is necessary to enable language from Corpus (languages are stored in Corpus in ISO format).

After I changed Snowball to work with ISO language codes, I also had to adapt the Preprocess Widget to store settings as ISO codes and call the Lemmagen filter with ISO language code.

Udpipe will be implemented in separate PRs.

Includes

• Code changes
• Tests
• Documentation

[PrimozGodec]

@VesnaT as #1025, you can make a workflow to test migration with tag 1.15.0 (git checkout 1.15.0) and open it with this change. It should work.

[codecov-commenter]

Codecov Report

Merging #1029 (aa3306c) into master (0495fd5) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1029      +/-   ##
==========================================
- Coverage   82.21%   82.20%   -0.02%     
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  Files          93       93              
  Lines       12294    12295       +1     
  Branches     1668     1670       +2     
==========================================
- Hits        10108    10107       -1     
- Misses       1877     1880       +3     
+ Partials      309      308       -1