Add a Fortran 90+ checker/styler

Like #408, but for Fortran 90 & later. https://github.com/pseewald/fprettify seems reasonable.

`spack` lacks a `py-fprettify` package, which would frustrate `uberenv` integration. Its only nontrivial dependency, `py-configargparse`, is already in `spack`, so remedying that situation should be straightforward.

Will try to get to this before December 1 because I want to use BLT for styling on a Fortran project.

Compare this request with issue #408, then review how BLT currently integrates checkers and stylers. Evaluate fprettify and the existing spack py-configargparse package; done means Fortran 90+ styling is exposed through BLT and the missing py-fprettify package supports uberenv integration.

Written by the indexing model from the issue text.