Add support for non-macOS Apple platforms

[HT154]

For non-macOS Apple platforms, this PR:

• Removes ServerMessageTransport (which should probably be renamed ServerExecMessageTransport eventually)
• Updates the default evaluator cache directory to be the application cache dir
• fatalError in EvaluatorManager methods that use Process or ServerMessageTransport

I've tested this by compiling for the iOS, tvOS, watchOS, and visionOS 26 simulators with deployment targets set as:

• iOS 16.6
• tvOS 16.6
• watchOS 9.6
• visionOS 1.3

I don't have the ability to test these on device and perform meaningful is-situ functional validation.

Hopefully resolves #15

[bioball]

Doesn't need to block this PR:

• Can we add CI tests that target each of these platforms?
• Need to update docs about using pkl-swift on non-macOS (eval first, then include the bundle)
• We should update https://github.com/apple/pkl-swift-examples with a build time eval example (just like we have for pkl-go-examples)

[pixlwave]

This is great to see, thank you!