Add Xcode 13.0 RC and Xcode 13.0

[macserv]

• Added checksums for Xcode 13.0 RC.
• Included a bit of minor polishing for language and presentation (e.g. s/OSX/macOS/, etc.).

Preview: https://github.com/macserv/apple-installer-checksums/blob/feature/add-xcode-13.0-rc1/xcode.md