Fix: Require phpstan/extension-installer

[localheinz]

This PR

• requires phpstan/extension-installer
• runs make static-code-analysis-baseline

Follows #139.

[codecov]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master   #149   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       41     41           
=======================================
  Files             4      4           
  Lines           159    159           
=======================================
  Hits            159    159

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