Add top-level xcodeproj (not symlink)

[bbrk24]

Checklist

• I searched the existing GitHub issues and this hasn't already been requested.

Problem

The xcodeproj file at the top level of the repository (_Pods.xcodeproj) is a symlink to a folder that's not checked in.

Feature Request

There should be a checked-in xcodeproj at the top level of the repository.

Value

This would have two advantages:

• the dochost script would no longer need to pod install in the iOS 13 example project;
• we would be able to support Carthage in addition to CocoaPods and SPM.

[bbrk24]

This is also causing Swift Package Index to incorrectly show Dependiject as only supporting Mac, not iOS, Apple Watch, or Apple TV.

[bbrk24]

I have an xcodeproj locally, but I accidentally based it off the branch for #84, not master, so this is unintentionally blocked by that.