phpstan fix: Scope generics

phpstan started failing with '... implements generic interface
Illuminate\Database\Eloquent\Scope but does not specify its types:
TModel'. We solve this by adding an implements docblock to the scopes
implementing that interface. They're fairly generic - we just use the
Model type itself in the code - so we use Model for the type parameter.

src/Database/Concerns/PendingScope.php

@@ -8,6 +8,7 @@ use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Scope;
 
+/** @implements Scope<Model> */
 class PendingScope implements Scope
 {
     /**