Resolve #3517506 "Phpstan validate level6"
https://www.drupal.org/project/admin_toolbar/issues/3517506
PHPSTAN: Raised validation level from 0 (min) to 6.
Changed 4 lines of module's code with very minor changes.
Fixed all validation errors, mostly:
- Updated Doc comment blocks return types.
- Updated Doc comment blocks param types.
- Added inline
@vartypes definitions. - Changed 4 lines of code, to add elvis operators and reorder chained calls.
- Caught a wrong variable passed in assert function in a Test class.
Added project's phpstan.neon file to raise validation level to 6 and ignore certain error messages.