@'verbatim \ strings' seem to not be highlighted properly

taking [the example from jsonnet.org](https://jsonnet.org/learning/tutorial.html) there's this bit in the middle:

```jsonnet
@'A clear \ red drink.'
```

with this grammar [babi](https://github.com/asottile/babi) (and probably vs code too) renders this as:

I assume it's supposed to look roughly like c#'s verbatim string highlighting:

Start with the Jsonnet grammar used by the VS Code extension and reproduce the example from the jsonnet.org tutorial: @'A clear \ red drink.'. Compare its highlighting with the shown C# verbatim-string behavior. Done means the verbatim string and its escaped content are highlighted consistently in the extension.

Written by the indexing model from the issue text.