feat: update to phpstan 1.0.0

[t-heuser]

Closes #15

[codecov]

Codecov Report

Merging #28 (7719359) into main (6352670) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #28   +/-   ##
=========================================
  Coverage     85.58%   85.58%           
  Complexity       38       38           
=========================================
  Files             7        7           
  Lines           111      111           
=========================================
  Hits             95       95           
  Misses           16       16           

Flag	Coverage Δ
tests	85.58% <ø> (ø)

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