fix: create Transformers on demand

[ianlewis]

Description:

Create the folding Transformer via a constructor on demand. A Transformer can hold state so it can't always be reused. It must be re-created each use.

Related Issues:

Fixes #87

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Update documentation if applicable.
• Add a note in the CHANGELOG.md if applicable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.

Project coverage is 41.09%. Comparing base (a3b74f9) to head (48bcf19).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
stardict.go	86.36%	2 Missing and 1 partial ⚠️
idx/idx.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   40.94%   41.09%   +0.14%     
==========================================
  Files          20       20              
  Lines        1165     1173       +8     
==========================================
+ Hits          477      482       +5     
- Misses        644      646       +2     
- Partials       44       45       +1     

Files with missing lines	Coverage Δ
syn/syn.go	40.86% <100.00%> (+1.97%)	⬆️
idx/idx.go	50.00% <85.71%> (+0.84%)	⬆️
stardict.go	43.69% <86.36%> (-0.86%)	⬇️

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