We make four adjustments in this RFC:
1. `{{` is not allowed. This is likely a valid but poor attempt at escaping coming from C#, Rust, or Python.
2. We now allow `` `this` `` with zero interpolating expressions.
3. We now allow `` f `this` `` also.
4. Explicitly say that `` `this` `` and `` `this {that}` `` are not valid type annotation syntax.