Add `println` for `Char`

[phischu]

We are currently lacking an overload of println for characters. I wonder what the behavior should be?

[jiribenes]

We do have a show instance from #1170 for Bytes, but not for Chars, good catch!
With characters, I think the issue is that it's potentially [?] backend-dependent.

[jiribenes]

We do have a show for Char though

effekt/libraries/common/effekt.effekt

Line 105 in 8b5005c

extern def showBuiltin(value: Char) at {}: String =

so any println: Char => Unit ought to act like printlnString . showBuiltinChar, right?