Add `statsExpressions` class and implement `print()` method

[codecov[bot]]

Codecov Report

Merging #219 (785fac8) into main (5920ae9) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 785fac8 differs from pull request most recent head 7348bfd. Consider uploading reports for the commit 7348bfd to get more accurate results

@@            Coverage Diff            @@
##              main      #219   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        14    +1     
  Lines          499       513   +14     
=========================================
+ Hits           499       513   +14     

Impacted Files	Coverage Δ
R/contingency_table.R	100.00% <ø> (ø)
R/meta_analysis.R	100.00% <ø> (ø)
R/oneway_anova.R	100.00% <ø> (ø)
R/two_sample_test.R	100.00% <ø> (ø)
R/add_expression_col.R	100.00% <100.00%> (ø)
R/pairwise_comparisons.R	100.00% <100.00%> (ø)
R/print.R	100.00% <100.00%> (ø)
R/tidy_model_expressions.R	100.00% <100.00%> (ø)

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