Enhancement: Require localheinz/phpstan-rules

[localheinz]

This PR

• requires localheinz/phpstan-rules
• enables the Localheinz\PHPStan\Rules\Classes\AbstractOrFinalRule rule for phpstan/phpstan
• removes a test asserting that source classes are abstract or final

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #105   +/-   ##
=========================================
  Coverage     72.45%   72.45%           
  Complexity       63       63           
=========================================
  Files             7        7           
  Lines           265      265           
=========================================
  Hits            192      192           
  Misses           73       73

---

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 dcf55c2...db7ce9c. Read the comment docs.