Fix: Add type and return type declarations

[localheinz]

This PR

• adds type and return type declarations to FieldDefinition

Follows #1.

[codecov]

Codecov Report

Merging #93 into master will not change coverage by %.
The diff coverage is 94.11%.

@@            Coverage Diff            @@
##             master      #93   +/-   ##
=========================================
  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% <88.88%> (ø)	36.00 <30.00> (ø)
src/EntityDefinition.php	95.45% <100.00%> (ø)	13.00 <7.00> (ø)
src/FieldDefinition.php	95.65% <100.00%> (ø)	6.00 <6.00> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0d02aa9...90f5326. Read the comment docs.