Multiline strings are not recognised

[jiribenes]

Currently, it is impossible to write multiline strings (other than in extern definitions) -- I believe that's a regression.

[b-studios]

I am not sure whether we ever supported that, but I guess we could.

At least in the last version it seems to be like today:

https://github.com/effekt-lang/effekt/blob/523e11bea6c862935c86e84f8660a4d88574b95d/effekt/shared/src/main/scala/effekt/Parser.scala#L733