update use observable to return unsub function (#471)

and add a get value in case it’s usable

going to use usSyncExternalStore
https://github.com/facebook/react/blob/f7ee804c2270b451087a697d181ef9f421e23f33/packages/use-sync-external-store/src/useSyncExternalStoreShimClient.js#L30

docs https://react.dev/reference/react/useSyncExternalStore

packages/react-sdk/src/useObservable.ts

@@ -81,10 +81,10 @@ export function useObservable<T>(
         if (!observable) {
             return
         }
-        const subscription = observable.subscribe(onSubscribe, {
+        const unsub = observable.subscribe(onSubscribe, {
             fireImediately: opts?.fireImmediately,
         })
-        return () => subscription.unsubscribe(onSubscribe)
+        return unsub
     }, [opts, observable, onSubscribe])
 
     const data = useMemo(() => {

packages/sdk/src/observable/observable.ts

@@ -26,7 +26,7 @@ export class Observable<T> {
     subscribe(
         subscriber: (newValue: T) => void,
         opts: { fireImediately?: boolean; once?: boolean; condition?: (value: T) => boolean } = {},
-    ): this {
+    ): () => void {
         const sub = {
             id: this._nextId++,
             fn: subscriber,
@@ -37,7 +37,7 @@ export class Observable<T> {
         if (opts.fireImediately) {
             this._notify(sub, this.value)
         }
-        return this
+        return () => this.unsubscribe(subscriber)
     }
 
     when(