Enhancement: Introduce named constructor and mark primary constructor private

[localheinz]

This PR

• introduced named constructors for field definitions and marks primary constructor as private

Related to #167.

[codecov]

Codecov Report

Merging #188 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #188      +/-   ##
============================================
+ Coverage     97.42%   97.55%   +0.12%     
- Complexity       86       91       +5     
============================================
  Files            19       19              
  Lines           233      245      +12     
============================================
+ Hits            227      239      +12     
  Misses            6        6              

Impacted Files	Coverage Δ	Complexity Δ
src/FieldDefinition.php	100.00% <100.00%> (ø)	6.00 <0.00> (ø)
src/FieldDefinition/Closure.php	100.00% <100.00%> (ø)	3.00 <2.00> (+1.00)
src/FieldDefinition/Reference.php	100.00% <100.00%> (ø)	3.00 <2.00> (+1.00)
src/FieldDefinition/References.php	100.00% <100.00%> (ø)	4.00 <3.00> (+1.00)
src/FieldDefinition/Sequence.php	100.00% <100.00%> (ø)	4.00 <3.00> (+1.00)
src/FieldDefinition/Value.php	100.00% <100.00%> (ø)	3.00 <2.00> (+1.00)

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