More dynamic f-interpolator with interpolated formatting directives

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.