Wrong highlighting of a block in splice

The following program:
```scala
import stream
import stringbuffer

def main() =
println("test: ${collectString { ['h', 'e', 'l', 'l', 'o'].each }}")
```

highlights wrongly:

I think the rough idea is to count braces inside of the splice -- maybe allow these as a separate top-level pattern that has higher priority than string interpolation, then everything might just work out ®.

Here's the relevant code:
https://github.com/effekt-lang/effekt-vscode/blob/e9241a1aa2b51b76c28fa5742e9b8891c7cbc721/syntaxes/effekt.tmLanguage.json#L245-L258