Following a75952cf77, this is one more upgrade to the widget communication that I'd like to make within this release cycle.
The motivating issue is https://github.com/element-hq/element-web/issues/29429. Fundamentally, without a 'join' action, the only info Element Web can use to determine whether it's joined the call is whether a MatrixRTC membership exists. But membership state events can inaccurately represent the client's actual state (whether because delayed events aren't supported, or because the delayed event hasn't timed out yet), so I suggest we send a 'join' action here just as we do in the Element Web Jitsi wrapper (e9a3625bd6/src/vector/jitsi/index.ts (L503)) to let Element Web tap directly into the widget's local state. (This will need additional Element Web changes, but is certainly backwards compatible.)
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