mechanims for extensions that require features in beta to declare it (and not load in live)

During development of features that require CODAL changes, we may end up with a situation where an extension only works with an updated CODAL, say in the beta release of MakeCode

At the moment, it doesn't seem like there's anything preventing these extensions from being loaded into the live editor and causing errors. As an example, here's an extensions developed in beta, that relies on the new sound blocks and latest CODAL:

https://github.com/jaustin/pxt-musicextras

Despite this section in pxt.json

...
    "targetVersions": {
        "branch": "v4.1.22",
        "tag": "v4.1.22",
        "commits": "https://github.com/microsoft/pxt-microbit/commits/87c494673712c7699f68564a13e1b634d20da326",
        "target": "4.1.34",
        "pxt": "7.5.14",
        "targetId": "microbit"
    },
...

The extensions still loads in live today, but has no blocks and looks wrong.

As we would like to be able to continue to user-test new features between annual MakeCode releases it would be great to have one of the following things happen:

• We can mark an extension as 'beta' which would mean it can only be used in the beta editor (perhaps only show up in beta editor?)
• we can have fine-grained control over the required versions and things like 'live' will refuse to load extensions that require things newer than live contains (and perhaps prompts to try /beta)
• Something else that stops extensions that we're testing being loaded into live where they won't work.

(perhaps this already exists and I don't know the flags, sorry if so!)

Start by examining the pxt.json targetVersions metadata and how the live and beta editors decide whether to load an extension. Define a compatibility or beta-only behavior that prevents extensions requiring newer CODAL or editor features from loading in live, and verify that the example extension is rejected or directed to beta.