Add PHPStan

[paul121]

WIP

Done! This is a start. Can continue improving PHPStan levels overtime

[paul121]

Aghh. I forgot that the strict types are causing some runtime errors on quick forms:

The website encountered an unexpected error. Try again later.

TypeError: Drupal\farm_rothamsted_quick\Plugin\QuickForm\QuickExperimentFormBase::getTermTreeOptions(): Argument #2 ($parent) must be of type int, string given, called in /var/www/html/web/modules/farm_rothamsted/modules/farm_rothamsted_quick/src/Traits/QuickTaxonomyOptionsTrait.php on line 47 in Drupal\farm_rothamsted_quick\Plugin\QuickForm\QuickExperimentFormBase->getTermTreeOptions() (line 75 of modules/farm_rothamsted/modules/farm_rothamsted_quick/src/Traits/QuickTaxonomyOptionsTrait.php).
Drupal\farm_rothamsted_quick\Plugin\QuickForm\QuickExperimentFormBase->getChildTermOptionsByName('log_category', 'Drilling categories') (Line: 380)
Drupal\farm_rothamsted_quick\Plugin\QuickForm\QuickExperimentFormBase->buildForm(Array, Object) (Line: 55)
Drupal\farm_rothamsted_quick\Plugin\QuickForm\QuickDrilling->buildForm(Array, Object) (Line: 114)
Drupal\farm_quick\Form\QuickForm->buildForm(Array, Object, 'drilling')

This one in particular might be an easy fix, but I'm a little worried about other cases that strict types might bite us...

[paul121]

Moving strict types to a separate PR #767