Fix: Use interface in type and return type declarations

[localheinz]

This PR

• uses Doctrine\ORM\EntityManagerInterface instead of Doctrine\ORM\EntityManager in type and return type declarations

[codecov]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master      #24   +/-   ##
=========================================
  Coverage     61.85%   61.85%           
  Complexity       80       80           
=========================================
  Files             8        8           
  Lines           194      194           
=========================================
  Hits            120      120           
  Misses           74       74

Impacted Files	Coverage Δ	Complexity Δ
src/EntityDef.php	0% <0%> (ø)	18 <1> (ø)	⬇️
src/FixtureFactory.php	93.82% <100%> (ø)	36 <1> (ø)	⬇️

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