Notify users about potential bugs with the upcoming Scratch editor update

[DNin01]

Scratch's next editor update will be big. As we probably all know, this "Scratch 4.0" will update Blockly to a newer version and change some things around within Scratch's internals, which will require us to update our addons eventually.

@mxmou has been very hardworking to add compatibility layers ahead of time. These measures will gracefully and automatically activate when the Scratch update is pushed. However, it's still a good idea to inform users that there may be breakage, even if we think we've checked everything. Something could change unexpectedly or we could've missed something. It's also nice to normalize issues with Scratch Addons and Scratch playing together.

I suggest a message like this:

We could also display a similar message on the next update notification. It could, for example, mention that the update contains some compatibility updates for an upcoming Scratch update.

[Samq64]

Let's be careful not to ship this too long before Scratch's update though.