Enhancement: Require localheinz/phpstan-rules

[localheinz]

This PR

• requires localheinz/phpstan-rules
• includes rules.neon from localheinz/phpstan-rules
• fixes issues detected via static analysis
• ignores an error related to an interface we don't control

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #109   +/-   ##
=======================================
  Coverage       100%   100%           
+ Complexity      136    129    -7     
=======================================
  Files            30     30           
  Lines           415    404   -11     
=======================================
- Hits            415    404   -11

Impacted Files	Coverage Δ	Complexity Δ
src/SchemaNormalizer.php	100% <ø> (ø)	32 <0> (-3)	⬇️
src/IndentNormalizer.php	100% <100%> (ø)	2 <1> (-1)	⬇️
src/FixedFormatNormalizer.php	100% <100%> (ø)	2 <0> (-1)	⬇️
src/AutoFormatNormalizer.php	100% <100%> (ø)	2 <1> (-1)	⬇️
src/Format/Formatter.php	100% <100%> (ø)	3 <1> (-1)	⬇️

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