"dart analyze" annotations?

I'm not sure if this is the appropriate place for it, but since this seems to be the only official GH action, maybe?

During a GitHub actions run, you can output special syntax to create warnings/errors that show up against locations of the source code so they're easy to see in the GitHub web app:

https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-a-warning-message

```
::warning file={name},line={line},endLine={endLine},title={title}::{message}
```

It could be nice if there was some kind of analyze action that uses `dart analyze --machine/--json` and formats like this so it's easy to get the annotations without writing your own custom code.

@devoncarew I thought I'd seen some code (possibly authored by you) that did this somewhere, but I was unable to find it 🙃

Review the setup-dart repository and the linked GitHub Actions workflow-command documentation. Compare dart analyze --machine/--json output with the warning and error annotation syntax, and determine how the action should expose this behavior. Done means analyzer diagnostics appear as source-location annotations in the GitHub web app.

Written by the indexing model from the issue text.