remove all messagesimulators

[thomasjball]

Once we are past release of pxt-microbit, we should do the following in order to remove the old messagesimulator framework

• move https://github.com/microsoft/pxt-arcade to new extension
• move https://github.com/microsoft/pxt-maker to new extension
• move https://github.com/microsoft/pxt-calliope to new extension
• move the robot simulator from pxt-microbit to the simx framework (used by pxt-microbit and pxt-calliope)
• look for other uses of messagesim and eliminate if possible (after above two items, there will be no uses of messagesim in the editors we support, I believe)
• remove messagesim logic (later)