Update dependency from apple/swift-syntax to swiftlang/swift-syntax

[oootkarsh]

It has been a while since apple changed the domain of this package from apple to swiftlang. While compiling an app, verge now throws a warning:

'verge' dependency on 'https://github.com/apple/swift-syntax.git' conflicts with dependency on 'https://github.com/swiftlang/swift-syntax' which has the same identity 'swift-syntax'. this will be escalated to an error in future versions of SwiftPM.

I believe we should update the dependency and not wait for SwiftPM to start throwing an error.