Allow injection via LiformInterface

[tacman]

Fetching the service via $this->get, as described in the README, has been removed in Symfony 6, as all services are private by default and the container and get() helper methods aren't available.

Instead, the documentation should reflect getting the service by injecting it in the contructor or the controller method, e..g

__construct(private Liform $liform)

Ideally, though, this would work with LiformInterface (the abstract instead of the concrete).