[vm] Add _nativeEffect top-level function

[dcharkes]

We should add a _nativeEffect top-level function which can be used to describe the effects of native calls in Dart code as one of its arguments. This enables us to describe arbitrary side-effects as long as they are expressible in Dart code, rather than for every new type of side effect introducing a new pragma.

This _nativeEffect should then be compiled to nothing in the backends, or be removed as the last step of the CFE.

Currently, we do without _nativeEffect, this does not express intent well. For example:

let #keepConstructorAlive = MyStruct.#fromTypedDataBase(Int8List(1)) in
dynamicLibrary.lookupFunction<MyStruct Function(), MyStruct Function()>('foo');

With _nativeEffect the intent is explicit:

let #keepConstructorAlive = _nativeEffect(MyStruct.#fromTypedDataBase(Int8List(1))) in
dynamicLibrary.lookupFunction<MyStruct Function(), MyStruct Function()>('foo');

Background internal doc: go/dart-ffi-struct-treeshaking