* Check native sliding sync support against an unstable feature flag
The `OPTIONS` approach from https://github.com/matrix-org/matrix-react-sdk/pull/12492 doesn't work because Synapse *always* responds with 204 (success) to `OPTIONS` requests, as described here: https://github.com/element-hq/synapse/issues/17153
We further can't use `HEAD` because it's not part of the allowed CORS methods, meaning the browser will mask the exact status code and error message from us, and the proxy hangs on the request anyways: https://github.com/matrix-org/sliding-sync/pull/429
To avoid these problems, we instead search for an unstable feature flag to be exposed by the server. Presence of this flag denotes native support. See https://github.com/matrix-org/matrix-spec-proposals/pull/3575/files#r1588877046 for details.
Implementations which support sliding sync natively will need to update to support this new unstable feature flag usage.
* Appease the linter
* Appease the tests
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