Add configuration for default configuration format

[tomasfejfar]

This would be a nice DX enhancement.

Steps to reproduce:

• have services.yml
• open any file with class inside
• Generate → Symfony service

Expected:

• code at the bottom is already YAML (or there is a configuration option to make it so)
• copy and insert code

Actual:

• you need to click to change format to YAML
• copy and insert code

[Haehnchen]

last selected output format is known written in settings and value reused