We've had this restriction in the original RFC, but decided to remove it afterwards. This change puts the restriction back - we need to work through some implications of future support for string-based DSLs together with interpolated strings which may or may not change the behavior here, for example to allow something like
```
local fragment = xml `
<img src="{self.url}" />
`
```
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.