fix!: exit success when adding an already-added plugin

[hyperupcall]

Summary

Closes #841

I do not consider this a breaking change because the fixed behavior does not break currently existing scripts that have the workaround (ie (exit 2) || : has the same affect as (exit 0) || :)

[jthegedus]

I agree with this change.

This is a breaking change as people have scripts dependent on the output here. It was added specifically because people wanted to know the difference of these return statuses.

I am updating the PR title conventional commit to reflect this

-fix:
+fix!:

---

I am open to further discussion here, but I find, for the most part, OSS libraries do a bad job of determining what is considered a public API and therefore what is considered "breaking". Exit codes and formatting of our return values are our public API, so I consider this type of change breaking.

Whilst guards such as (exit 2) || : should be used when consuming the return code, they may not be and that is somewhat on us. Despite being pre 1.0.0 and semver technically allowing us to break everything as much as we want, we should indicate to external users as best as possible. IMO this means this should be considered breaking.

[jthegedus]

Thanks, LGTM. Finally! 😌