Enhancement: Add type declarations

[localheinz]

This PR

• adds type declarations

[codecov]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #2   +/-   ##
=========================================
  Coverage     96.07%   96.07%           
  Complexity       25       25           
=========================================
  Files             1        1           
  Lines            51       51           
=========================================
  Hits             49       49           
  Misses            2        2

Impacted Files	Coverage Δ	Complexity Δ
src/JsonFormatter.php	96.07% <100%> (ø)	25 <25> (ø)	⬇️

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