Add hyphens property and test that cover it

[matiasgiorda1999]

Fixes #53

[codecov]

Codecov Report

Merging #54 (56349dc) into release-candidate (aec4730) will not change coverage.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           release-candidate       #54   +/-   ##
===================================================
  Coverage             100.00%   100.00%           
===================================================
  Files                    118       118           
  Lines                   4590      4595    +5     
===================================================
+ Hits                    4590      4595    +5     

Impacted Files	Coverage Δ
source/RenoirSt-Core/CssDeclarationBlock.class.st	100.00% <100.00%> (ø)
...ce/RenoirSt-Tests/CssDeclarationBlockTest.class.st	100.00% <100.00%> (ø)

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