problem with simmessages.send logic

This problem applies to both pxt-microbit and pxt-arcade. The following code will cause a second instance of a mbit/arcade simulator to appear:

const data = Buffer.fromUTF8("hello")
control.simmessages.send("foobar", data)

This is because all simmessages sent the above way have
the broadcast bit set, which is used to determine if a
new simulator can be created; if you send a message that doesn't match
to a simulator extension ("jacdac", "robot", etc.)
you will get a second mbit simulator

Repro with arcade: https://makecode.com/_38FRkP531hjj
Repro with mbit: https://makecode.microbit.org/_ETPMjLM6D717

The send function is declared as follows in pxt-common-packages/libs/base/controlmessage.ts:

    //% shim=pxt::sendMessage
    export declare function send(channel: string, message: Buffer, parentOnly?: boolean) : void;

The default value for parentOnly is false; if it is set to true, no new simulators will be created at all.

the implementation is in pxsim.pxtcore as

    // general purpose message sending mechanism
    export function sendMessage(channel: string, message: RefBuffer, parentOnly?: boolean) {
        if (!channel) return;

        Runtime.postMessage({
            type: "messagepacket",
            broadcast: !parentOnly,
            channel: channel,
            data: message && message.data
        } as SimulatorControlMessage)
    }

Here we can see how the broadcast bit is set based on !parentOnly.