[WASM] Investigate removing `System.Native.TimeZoneData.Invariant`

Since we have a feature switch guarding the use of the only native method that provides the data, if the managed linker is able to eliminate that native method reference from managed code, then the native linker won't bring in the data as well, meaning we would be able to fold `System.Native.TimeZoneData` into `System.Native` proper.

Implementing this would also need a test: build with invariant timezone, then build without and compare the size difference (should be about ~300kb). Note this requires actually using the relevant time zone APIs.