5985: Make MergeBehaviour configurable r=jonas-schievink a=Veykril
This should make the newly implemented `MergeBehaviour` for import insertion configurable as roughly outlined in https://github.com/rust-analyzer/rust-analyzer/pull/5935#issuecomment-685834257. For the config name and the like I just picked what came to mind so that might be up for bikeshedding.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>