Add XcodePlugin support

[p4checo]

Replicating upstream SwiftGen#4 in our fork until it gets merged 🕯️

This will allow us to have a git tag which then produces a "stable" version so SPM doesn't complain with:

Dependencies could not be resolved because '<x>' depends on '<y>' 0.1.0..<0.2.0.
'<x>' >= 0.1.0 cannot be used because package '<y>' is required using a stable-version but '<y>' depends on an unstable-version package 'swiftgenplugin' and no versions of '<y>' match the requirement 0.1.1..<0.2.0.