Detect when a user is missing a language extension

[pokey]

If a user tries to run a Cursorless command in a file type whose file extension is not registered (eg .scm or .talon), they'll get a strange error message saying "language plaintext is not supported". We'd like to detect this case by inspecting the file extension and showing an error message suggesting to install an extension that registers the given file extension

[AndreasArvidsson]

This will be easier to do once we have gotten rid off the legacy syntax scopes. Then we can do this in the scope handler factory. For now we have fallbacks to legacy in a few places.