Don't overwrite shared schemes

Currently if you define schemes in your project configuration, they will be created as shared schemes. This also overwrites any existing shared schemes. The problem is that if you create a scheme in the Xcode UI, it defaults to share so if a user forgets to uncheck that when creating a custom scheme, their schemes will be overwritten the next time they generate.

I think ideally any shared schemes in XcodeGen would just be added along side any currently existing shared schemes (unless they have the same name, at which point they should be overwritten)

The issue does not name a file or test. Start by tracing how project configuration creates shared schemes, then check the generated project against existing shared schemes. Done means existing schemes remain unless an XcodeGen scheme has the same name, in which case it is replaced.