`FloatFormat` should allow scientific notation with given precision

Hello!

I was wondering if we could get the possibility to format a Float or Double in scientific notation with a given precision (number of decimal places). That is:

```
scientificFixedPrecision :: Int -> FloatFormat
```

Further, it would make sense to format Float and Double in standard notation, but with a given number of significant digits.

Start by locating the FloatFormat entry point and the existing Float and Double formatting behavior. Clarify the API and expected output for scientific notation with a fixed number of decimal places, plus standard notation with significant digits; done means both requested formatting capabilities are implemented and covered by tests.

Written by the indexing model from the issue text.