More dynamic f-interpolator with interpolated formatting directives

[som-snytt]

A frequent need:

val prefix = String.format(s"%${offset - 2}s |", lineNbr)

written as

f"$lineNbr%${offset - 2}s |"

At compile time, it's only necessary to verify that the interpolation in the formatting directive is an int.

Perhaps other interpolated directives would be tricky to verify and might be rejected.