Fix disconnect on onmount regression due to strict mode

[lukasIO]

Fixes #111

React 18's strict mode calls all useEffect hooks twice.
This generally leads to the the useEffect cleanup function being called on a component, even though it never unmounted.

Within useRoom in this package, the room state was initialized with useState(new Room()).
Because we want the LiveKitRoom component to automatically disconnect when unmounted, this lead to a disconnect call being called on the only existing instance of room, when strict-mode's second hook execution invoked the cleanup function. Because this was happening while the room was already connecting it made it effectively impossible to connect to a room in dev mode.
A previous attempt to fix this caused the auto-disconnect logic not to fire at all on component unmount.

This fix re-establishes auto-disconnect behaviour and moves room instantiation into a useEffect hook in order to prevent disconnect being called on the only present room instance.

This changes the assumption that useRoom() always directly returns a valid Room object. With this change it can also happen that room is undefined on the first render, before the useEffect has set its value.

[changeset-bot]

🦋 Changeset detected

Latest commit: c236376

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@livekit/react-components	Minor
@livekit/react-core	Minor

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[davidzhao]

good fix!