Fix: Rename FieldDef to FieldDefinition

[localheinz]

This PR

• renames FieldDef to FieldDefinition

Follows #1.

[codecov]

Codecov Report

Merging #92 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #92   +/-   ##
=========================================
  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/FieldDefinition.php	95.65% <ø> (ø)	6.00 <0.00> (?)

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