Add SPM support to powersync_flutter_libs

[simolus3]

For native dependencies on Apple platforms, Flutter is migrating to Swift package manager to replace CocoaPods.

SPM support is experimental at the moment (users have to opt-in with flutter config --enable-swift-package-manager). But since we can support both in powersync_flutter_libs, we probably should:

• We're still early, but CocoaPods trunk will become read-only in 15 months. So we have to migrate all our SDKs eventually.
• Users have a similar interest, and they can only switch to SPM after all their dependencies have. So this unblocks users.

So, this PR:

• merges ios and macos podspecs for powersync_flutter_libs into a single darwin directory for simplicity.
• creates a new SPM package for powersync_flutter_libs depending on our core extension dependency.
• updates demos/supabase-todolist-drift to use SPM exclusively.
• updates demos/supabase-todolist to use CocoaPods (and explicitly opt out of SPM) so that we can keep testing both variants of powersync_flutter_libs.

I ran both updated projects on macOS and an iOS simulator for verification.

[rkistner]

Happy with the changes in general - would be great to not depend on CocoaPods anymore!

I see some of the platform support versions have changed - are these differences intentional?

1. ios: 11.0 in podspec, now 12.0 in podspec, 13.0 in Package.swift
2. osx: Was 10.11 in podspec, now 10.14 in podspec, 10.15 in Package.swift

[simolus3]

I see some of the platform support versions have changed - are these differences intentional?

I think we could keep referencing the older versions as well, but iOS 13 and macOS 10.15 are the minimum versions supported by Flutter. I agree that we should make these consistent though.