Support comments in multiline expressions
The format I'm parsing requires me to write a [monster ternary expression](https://github.com/mickdekkers/tesv-ksy/blob/a3f2e61fa61c70457a7d2fdbaec78eda6584fc53/ess.ksy#L622-L683), but it's really difficult to read without comments. It would be great if `#` comments were supported in multiline expressions.