Add macOS native targets (macosX64, macosArm64)

[dima-dencep]

Adds macosX64() and macosArm64() to the Compose Multiplatform library convention plugin so ConfettiKit publishes klibs for those targets.

Heads-up on x86_64 Apple targets

This PR adds macosX64 alongside macosArm64 to keep parity with the existing iosX64 target. All Apple x86_64 targets share the same deprecation trajectory:

• Kotlin 2.2.20: demoted to tier 2
• Kotlin 2.3.0: demoted to tier 3 (iosX64, macosX64, tvosX64, watchosX64)
• Kotlin 2.3.20: macosX64, tvosX64, watchosX64 marked deprecated
• Kotlin 2.4.0: planned removal of x86_64 Apple targets (~September 2026)

When you decide to drop iosX64, dropping macosX64 in the same release makes sense. Until then, supporting both is consistent.

Refs:

• https://kotlinlang.org/docs/native-target-support.html
• https://blog.jetbrains.com/kotlin/2023/02/update-regarding-kotlin-native-targets/