Add a Fortran 90+ checker/styler

[goxberry]

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.

[goxberry]

Related upstream spack PR for py-fprettify: https://github.com/spack/spack/pull/19699