chore: updates for Xcode 15.2

[stigi]

What's in this PR?

This PR updates two dependencies to make them compile with Xcode 15.2:

• Nimble (used for tests, and only referenced via Carthage, but not via Cocoapods spec and SwiftPM manifest)
• Ably

The errors appeared when trying to install the development dependencies for working on MagicBell.xcodeproj. For this one has to first run carthage bootstrap --use-xcframeworks --no-use-binaries which resulted in these two errors (line breaks added for readability):

/Users/ullrich/Projects/magicbell-swift/Carthage/Checkouts/Nimble/Nimble.xcodeproj: warning: 
The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.10, but the range
of supported deployment target versions is 10.13 to 14.2.99. (in target 'Nimble-macOS' from project 'Nimble')
** ARCHIVE FAILED **

/Users/ullrich/Projects/magicbell-swift/Carthage/Checkouts/ably-cocoa/Ably.xcodeproj: warning:
The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.12, but the range
of supported deployment target versions is 10.13 to 14.2.99. (in target 'Ably-macOS' from project 'Ably')
** ARCHIVE FAILED **

Xcode versions 14 and above only support a minimum deployment target of 10.13 (documented here).

Drive By changes

• Fixed a few typos in the readme
• Added a test target to the swift package manager manifest (allows to run unit tests via swift test from the CLI)

Test Plan

Carthage

To reproduce the issue and verify the fix make sure to have Carthage installed and follow these steps:

• Clone the project
• Run the following command in the projects root: carthage bootstrap --use-xcframeworks --no-use-binaries (⚠️ this will fail without the changes in this PR for Nimble and Ably)
• 🍵 wait until the Carthage dependencies finished building
• Open MagicBell.xcodeproj
• Select the MagicBellTests target and a Simulator as a testing device
• Press ⌘+U to build and run tests

Cocoapods

Tested by running the Example app that integrates the SDK via Cocoapods.

SwiftPM

Added a unit test target to SPM and ran the tests via swift test