usability issue: Custom pretty-printer creator silently overrides setAutoImports

Hi!

When you set a custom pretty-printer creator (i.e. with `Environment.setPrettyPrinterCreator`), this silently overrides the effect of `Environment.setAutoImports` in the `StandardEnvironment`. https://github.com/INRIA/spoon/blob/8a2cce96f45d74d6cc6b3d678947cee28cfdf991/src/main/java/spoon/support/StandardEnvironment.java#L661-L693

I've seen code setting both auto-imports and a custom creator multiple times now, for example here in Sorald: https://github.com/SpoonLabs/sorald/blob/2434d1d09812a40b3241adf942ad7b7bf4e0c824/src/main/java/sorald/SoraldAbstractRepair.java#L46-L65

I've myself made the mistake of thinking that I could use the sniper printer _and_ auto-imports. I see two reasonable options for improving this:

1. Simply issue a warning if both a custom pretty-printer creator and auto-imports are set, saying that the one overrides the other.
2. Prepend or append the auto-import processors to the preprocessors of the printer created by the pretty-printer creator.

I would personally prefer option 2, but I also see how that somewhat conflicts with the idea of setting a custom pretty printer and may cause breakage in existing applications.

Any thoughts on this?