Chore: add phpstan for static analysis

[fadrian06]

What kind of change does this PR introduce? (pls check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe below

Description

• Add phpstan static analyzer
• Remove unused $testUser in user.test.php
• Fix @param names in Leaf\Auth methods
• Fix inconsistent cases of Leaf\Db
• Ensure $sessionLifetime is an integer for + operation without warnings.
• Fix @param names in Leaf\Auth\UsesRoles methods
• Removed undefined @param $subscription on Leaf\Auth\UsesSubscriptions::subscription method.
• Set null as default value for the Leaf\Auth::$user property.
• Simplified checks of $this->user() in Leaf\Auth::middleware
• Add leafs/leaf, leafs/billing and league/oauth2-google as dev dependencies, for intellisense.
• Improve array docblocks and add all return types.
• Remove implicit false to string conversion in Leaf\Auth\User::__toString method.
• Add checks for unset keys of Leaf\Auth\UsesSubcriptions::$subscriptions property.
• Fix typo in Leaf\Auth\UsesSubscriptions::cancelSubscription method.

Does this PR introduce a breaking change? (check one)

• Yes
• No