Filter out linked issue when creating alert rules

### Problem Statement

We love that we can configure Sentry to alert us in Slack! However the alert rules don't quite fit our issue triaging workflow

Whenever a new issue appears in Sentry, we link it to an issue in our task tracker. At that point, we no longer want Sentry to alert us in Slack, but we don't have a way of excluding linked issues

### Solution Brainstorm

In other parts of the product, it's possible to filter by `is:linked`. Is it possible to include that as a filter option here?

### Product Area

Alerts