Finishing the Fractional Differential Equation (FDE) interface

[ChrisRackauckas]

#3776 merged as a major leap towards fractional differential equations support via the algorithm in https://arxiv.org/pdf/2506.04188. But we still need to make it use the standard front end. This would mean:

• Support for defining fractional differentials, i.e. D(x)^0.5 ~ x should "just work"
• Make this happen during the normal mtkcompile pipeline, i.e. mtkcompile on a System where some of the equations have fractional derivatives should run the expansion code and define the appropriate observed equations so that this all "just works".

[ChrisRackauckas]

@fchen121