WebHID: methods which write (`sendReport`) to the device should be async about xkeys HOT 1 OPEN

After some consideration, I propose these changes to address this:

• Add a flush(): Promise<void> method to wait for any lingering writes.
• Also: Once close() is called, promise to never emit any errors anymore.

With these changes, you should be able to acheive a proper cleanup like so:

const xKeys = setupXKeysPanel()
const errorListener = (e) => console.error(e)
xKeys.on('error', errorListener)

xKeys.setAllBacklights(false)

// cleanup:
await xKeys.flush() // waits for all writes to finish
await xKeys.close() // close the device.
xKeys.off('error', errorListener)

from xkeys.