Add proper setters and getters to `OutputFormat`

[oliverklee]

We should add proper getters and setters for the properties of OutputFormat. This should be covered with unit tests similar to this one: https://github.com/oliverklee/ext-oelib/blob/main/Tests/Unit/Domain/Model/GermanZipCodeTest.php#L48

Adding setIndentation will also fix a PHPStan warning, i.e., we'll need to regenerate the baseline then using composer phpstan:baseline.

When the last property is covered accordingly, we should remove the get and set methods as well as __call.

This should be split in multiple PRs in order to keep the PRs small, e.g., one property at a time.