Enhancement: Extract FieldDefinition\Reference

[localheinz]

This PR

• extracts FieldDefinition\Reference

[codecov]

Codecov Report

Merging #157 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #157      +/-   ##
============================================
+ Coverage     98.14%   98.17%   +0.02%     
- Complexity       75       77       +2     
============================================
  Files            13       14       +1     
  Lines           216      219       +3     
============================================
+ Hits            212      215       +3     
  Misses            4        4              

Impacted Files	Coverage Δ	Complexity Δ
src/EntityDefinition.php	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
src/FieldDefinition.php	100.00% <100.00%> (ø)	10.00 <1.00> (ø)
src/FieldDefinition/Reference.php	100.00% <100.00%> (ø)	2.00 <2.00> (?)
src/FixtureFactory.php	98.13% <100.00%> (ø)	37.00 <0.00> (ø)

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