Adding EnzymeRules

Similar to the ChainRulesCore support, we could use `EnzymeCore.EnzymeRules` to define `Forward` and `Reverse` mode rules for Enzyme in an extension.

EnzymeCore requires at least Julia v1.6. Making it a dependency for Julia versions older than v1.6 (as is done with ChainRulesCore) would then only be possible if AbstractFFTs add a Julia v1.6 version bound. But since EnzymeCore's sole dependency (Adapt) depends on Requires, it may make more sense to conditionally load on pre-v1.9 using Requires.jl: https://pkgdocs.julialang.org/dev/creating-packages/#Requires.jl

Unlike ChainRulesCore support, it probably only makes sense to only define rules for `StridedArray` inputs to avoid doing the wrong thing for sparse or structured arrays.