Fix: Add type declaration

[localheinz]

This PR

• adds a type declaration

Follows #1.
Follows #93.

🤦‍♂

[codecov]

Codecov Report

Merging #96 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #96   +/-   ##
=========================================
  Coverage     93.77%   93.77%           
  Complexity       77       77           
=========================================
  Files             9        9           
  Lines           209      209           
=========================================
  Hits            196      196           
  Misses           13       13           

Impacted Files	Coverage Δ	Complexity Δ
src/FixtureFactory.php	90.80% <100.00%> (ø)	36.00 <4.00> (ø)

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