Support exporting .ysls files

[janhohenheim]

No description provided.

[simonhdickson]

I have this working in a project I'm working on. It works fine for functions as they implement this:

YarnSpinner-Rust/crates/core/src/yarn_fn/function_wrapping.rs

Lines 57 to 60 in 8918900

	/// The [`TypeId`]s of the parameters of this function.
	fn parameter_types(&self) -> Vec<TypeId>;
	/// The [`TypeId`] of the return type of this function.
	fn return_type(&self) -> TypeId;

But commands don't so so I have to implement my commands in quite a hacky way:

YarnSpinner-Rust/crates/bevy_plugin/src/commands/command_wrapping.rs

Lines 158 to 164 in 8918900

	/// A type-erased [`YarnCommand`] as it appears in the [`YarnCommands`].
	pub trait UntypedYarnCommand: Debug + Send + Sync + 'static {
	#[doc(hidden)]
	fn call(&mut self, input: Vec<YarnValue>, world: &mut World) -> Box<dyn TaskFinishedIndicator>;
	#[doc(hidden)]
	fn clone_box(&self) -> Box<dyn UntypedYarnCommand>;
	}

I had a look at trying to make this change but I haven't quite figured out how to update the macro correctly.