Better support typed properties

Upgrades the underlying squirrelphp/strings component. This
removes compatibility to diverging property paths defined
in the form, we now only support direct properties which are
mapped 1:1 in the form.

Note that if filter annotations are used without direct mappings,
this might lead to errors and/or not work anymore.

The PHP 8 support is preliminary, as it was not tested yet.

Author: iquito

composer.json

@@ -22,7 +22,7 @@
     "require": {
         "php": ">=7.4",
         "symfony/dependency-injection": "^5.0",
-        "squirrelphp/strings": "^0.8.5"
+        "squirrelphp/strings": "^0.9"
     },
     "require-dev": {
         "bamarni/composer-bin-plugin": "^1.3",