add a 'preview tutorial' command

[jwunderl]

Add a 'preview tutorial in blocks editor' command to preview tutorials similar to https://makecode.com/tutorial-tool

Do we know if there's any way to set state for the webview directly from src/web/editor.ts? Ideally we should be persisting / reloading that tutorialUri from the state in MakeCodeEditorSerializer but I didn't see an obvious way to do that; is it just adding another message in resources/editorMessaging similar to the "open" one that sends in the tutorial uri and then doesn't propagate it to the editor frame?

I poked around for a minute to see about making it load in the current project as an extension, but it appeared to get dropped at some point in the tutorial load process and I didn't want to go down that rabbit hole for now. Here's the very quick branch as a ref https://github.com/microsoft/pxt/tree/allowLoadingHeaderForTutorialPreview. FWIW it looks like the github integration feature is broken as well microsoft/pxt-microbit#4777.

Maybe more importantly, it's a weird inconsistency we should probably think on for pxt tutorials -- if you load a tutorial from a github project it auto adds itself as a dependency (without declaring it in package.json), but it doesn't do the same thing if you load from a share link -- I've seen a few times where a tutorial author had a few lines in main.ts and were confused by how their project was misbehaving. I'd probably lean towards share link behavior of no magic imports personally (which this handles :) )

[riknoll]

IMO we should match the behavior of the github scenario, since that's the primary one that people will be using this with (not share links). Also, I think the state has to be set inside the iframe, so doing a custom message for the serializer would be the way to go