Add --skip-existing to publish command

[cnpryer]

While working on #759 I noticed that we didn't offer support for dispatching --skip-existing from Twine.

  --skip-existing       Continue uploading files if one already exists. (Only valid when uploading to PyPI. Other
                        implementations may not support this.)

Edit: This PR adds --skip-existing to the publish command.

This PR adds --force (-f) which covers dispatch of --skip-existing. I landed on "force" because I felt it was more generic. --skip-existing to me falls under the category of forcing behavior. So I decided to start with something more generic to approach this feature.

I'm kind of curious in general of --skip-existing, since, to me (IIUC), it's effectively a swap of an already published package -- which just feels odd to be able to do.

I tested this manually against upload.pypi.org. I was pretty sure that test.pypi.org implemented the same rejection, but I couldn't get it to trigger.

xlcsv on  master [!] is 📦 v0.1.0 via 🐍
❯ rye publish -y --skip-existing
Uploading distributions to https://upload.pypi.org/legacy/
WARNING  Skipping xlcsv-0.1.0-py3-none-any.whl because it appears to already exist                                             
WARNING  Skipping xlcsv-0.1.0.tar.gz because it appears to already exist  

Other than the manual testing, I've held off on adding anything since I've already added a bunch of testing in #759.

[mitsuhiko]

Force is kind of odd because it does not actually have the capability to replace the release. Not sure what a better flag is, but --force seems wrong. Maybe --ignore-existing is a reasonable flag?

[cnpryer]

You're right. I should have looked at this more closely. --skip-existing is a reasonable flag then. I'll use --skip-existing to align and if we want something else I can make that change here.

I'm not sure I see any value in --ignore-existing over --skip-existing since it's not a swap like I originally assumed. That was just my misunderstanding.