Support with old Flutter versions

[JustinDMH]

I jump back and forth between new Flutter SDKs and projects with old SDKs regularly. Is it possible to add in a check for the Flutter SDK of the current project?

If project SDK > 2.0, use null safety json, otherwise use the old snippets.json

The fix for now is to go into the extension and manually switch the version back and forth, but it can be cumbersome if done regularly.