Add rule for casting objects to string

[rvanvelzen]

Rule extracted from #1905 because it's useful without explicit stringable-object support as well

[ondrejmirtes]

I feel like we don't need a new rule here.

We need ObjectWithoutClassType::toString() to return ErrorType.

ObjectType already does this correctly.

Also, MixedType::toString() should be looked into for subtracted types.

[ondrejmirtes]

Nice! I also love this error:

 ------ ---------------------------------------------------------------- 
  Line   lib/Doctrine/ORM/Query/Expr/Base.php                            
 ------ ---------------------------------------------------------------- 
  104    Parameter #2 $array of function implode expects array<string>,  
         array<int, object|string> given.                                
 ------ ---------------------------------------------------------------- 

[ondrejmirtes]

Thank you!