Better return type for mode estimation

[mhauru]

The Optim.jl interface returns a ModeResult, a type we define for holding the outcome of the optimisation. For compatibility the Optimisation.jl interface currently returns the same type, but it's not quite a natural fit for it. We might consider changing the return type before making the switch of interfaces official and possibly deprecating the Optim.jl one. Might also consider aligning with similar types from SciML.

This came up in review of #2221 with @torfjelde, https://github.com/TuringLang/Turing.jl/pull/2221/files#r1608363430.