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}" />
`
```
This was implicitly assumed to be supported, but we should really just say so explicitly. If we didn't, there'd be two ways to interpret it: string interpolations are raw strings and escape sequences don't exist for the most part (false), or string interpolations are like strings and escape sequences do exist (true).
Also talks about the ambiguity with `\u{` and that it is defined to take priority and does not terminate the interpolation chunk and starts a new expression, instead it must be a well-formed escape sequence `\u{...}`. It is very unlikely there'd be another escape sequence that also has this same ambiguity problem, so we don't need to worry about this.
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.
