Refactor extension proposals check to use Array.prototype.includes

**Problem/Feature Description**
In the extension proposal utility, the code currently uses `existing.indexOf(e.id) === -1` to filter known extensions.

**Proposed Solution**
Replace `existing.indexOf(e.id) === -1` with modern TypeScript/JavaScript syntax using `!existing.includes(e.id)` to improve code readability and maintainability.