Fix: Verify that class implements interface

[localheinz]

This PR

• verifies that a candidate class implements Definition, instead of that it is a subclass of Definition

[codecov]

Codecov Report

Merging #299 into main will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #299   +/-   ##
=========================================
  Coverage     98.54%   98.54%           
  Complexity      100      100           
=========================================
  Files            23       23           
  Lines           274      274           
=========================================
  Hits            270      270           
  Misses            4        4           

Impacted Files	Coverage Δ	Complexity Δ
src/Definitions.php	91.66% <100.00%> (ø)	10.00 <0.00> (ø)

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