`bin/tapioca dsl --only` should not purge existing directory

[KaanOzkan]

I believe --only is mostly used as a debugging tool to test specific compiler output. However, its output is hard to understand since tapioca purges the files that aren't generated and creates a large diff

tapioca/lib/tapioca/commands/abstract_dsl.rb

Line 88 in 5a9d90c

files_to_purge = existing_rbi_files - generated_files

We could skip the purging if --only is present. Am I missing any valid use cases where --only will be used permanently by a repo? In that case we could have a separate flag to skip purging.