Expose WASM methods for getting and setting signal and data values

[jonmmease]

Adds several public Vega View methods to the MsgReceiver WASM struct:

pub fn get_signal(&self, name: &str, scope: &[u32]) -> JsValue ;

pub fn get_data(&self, name: &str, scope: &[u32]) -> JsValue;

pub fn set_signal(&self, name: &str, scope: &[u32], value: JsValue);

pub fn set_data(&self, name: &str, scope: &[u32], value: JsValue);

pub fn get_state(&self) -> JsValue;

pub fn set_state(&self, state: JsValue);

pub fn run(&self);

pub fn run_async(&self) -> Promise;

These are to make it easier to introspect and manipulate the view from outside.