Support `visionOS`

We'd like to support the visionOS as SDK option when compiling with XCode.

This should probably ~~either~~ be another `OS` ~~or another `IOSSdk`~~:

https://github.com/dart-lang/native/blob/876f9a1948c1642a57640ef9b1526746042a17eb/pkgs/native_assets_cli/lib/src/api/target.dart#L36-L42

https://github.com/dart-lang/native/blob/876f9a1948c1642a57640ef9b1526746042a17eb/pkgs/native_assets_cli/lib/src/api/ios_sdk.dart#L10-L12

We need this in:

* [ ] `package:native_assets_cli` so that it can be passed in from flutter_tools and consumed in `build.dart` scripts.
* [ ] `package:native_assets_builder` so that it can be passed in from flutter_tools.
* [ ] `package:native_toolchain_c` so that the right SDK path is passed when compiling C code.

Start with the linked target.dart and ios_sdk.dart definitions, then trace how SDK options move through package:native_assets_cli, package:native_assets_builder, and package:native_toolchain_c. Confirm how the option should be represented, then verify that visionOS can be passed from flutter_tools and selects the correct SDK path for C compilation.

Written by the indexing model from the issue text.