linear svm models

[EmilHvitfeldt]

This issues #35 talks about linear SVM models but we only have svm_poly() and svm_rbf().

I can see how svm_linear() would be a special case of svm_poly() but is there still room for svm_linear() for models that doesn't do polynomials?

[juliasilge]

Still TODO here is to add kernlab as an engine for svm_linear()

[github-actions]

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