Upgraded tests.yml to new PHP version, and added PHPstan.

[hexydec]

Fixed caching issue and added codecov token.
Removed unneeded check from htmldoc::find().
Fixed PHPstan issues.
Updated dependencies.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.71%. Comparing base (438f932) to head (8066c12).
⚠️ Report is 36 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff              @@
##             master      #14      +/-   ##
============================================
+ Coverage     94.32%   95.71%   +1.38%     
  Complexity      486      486              
============================================
  Files             8        8              
  Lines          1005     1003       -2     
============================================
+ Hits            948      960      +12     
+ Misses           57       43      -14     

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