add --delete/-d flag to the CLI

would be nice to be able to remove all the generated files. Like for example when I am tweaking my config, maybe I rename a few things and now I have to go in and manually delete hundreds of generated files.
Thankfully I have git, so I can remove them easily in there, but there are situations where having a delete flag on the CLI would be useful. Possible use cases:
- you don't have git,
- you have git, but you have other changes you want to keep without stashing them
- you have git, but you need to switch from a normal mode to `near-operation-file` and you want to prepare a commit that cleans the current generated files before making the config change in codegen.yml