Enhancement: Implement Specification::closure()

[localheinz]

This pull request

• implements Specification::closure()

[codecov]

Codecov Report

Merging #56 (7d98501) into main (684132e) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 7d98501 differs from pull request most recent head cd366c9. Consider uploading reports for the commit cd366c9 to get more accurate results

@@             Coverage Diff             @@
##                main       #56   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        36        37    +1     
===========================================
  Files              5         5           
  Lines             99       101    +2     
===========================================
+ Hits              99       101    +2     

Impacted Files	Coverage Δ
src/Specification.php	100.00% <100.00%> (ø)

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