Forward (::Machine).fitresult to RecipesBase

[MilesCranmer]

@ablaom

Apparently this is the idiomatic way to set up plotting recipes on custom types: https://docs.juliaplots.org/latest/RecipesBase/api/#RecipesBase.@recipe-Tuple{Expr}

With this change, a downstream package will be able to define:

@recipe function plot_my_regressor(fitresult::MyFitresultType)
    ...
end

and enable custom plotting of their fitted model.

---

Confirmed that this works for SymbolicRegression's regressors!

---

Fixes JuliaAI/MLJModelInterface.jl#188

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (081fec8) 88.05% compared to head (ad03525) 87.95%.

Files	Patch %	Lines
src/plots.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #951      +/-   ##
==========================================
- Coverage   88.05%   87.95%   -0.10%     
==========================================
  Files          27       28       +1     
  Lines        2519     2540      +21     
==========================================
+ Hits         2218     2234      +16     
- Misses        301      306       +5     

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[ablaom]

Looks good. Thanks for this contribution, @MilesCranmer.