feat: add config for inserting must_use in `generate_enum_as_method`
Should fix#13312
Didn't add a test because I was not sure on how to add test for a specific configuration option, tried to look for the usages for other `AssistConfig` variants but couldn't find any in `tests`. If there is a way to test this, do point me towards it.
I tried to extract the formatting string as a common `template_string` and only have if-else for that, but it didn't compile :(
Also it seems these tests are failing:
```
test config::tests::generate_config_documentation ... FAILED
test config::tests::generate_package_json_config ... FAILED
```
Can you also point me to how to correct these 😅 ( I guess there is some command to automatically generate these? )
