defaults for `add_tailor(method)` and `add_tailor(prop)`

[simonpcouch]

re:

https://github.com/tidymodels/workflows/blob/9e6ca982f8cd381cf4601cceb64f5ad323737476/R/fit.R#L69-L81

Should one be able to fit a workflow by itself without specifying these two? tune can handle method and prop in a principled way internally since it has access to the splits, but workflows doesn't know where fit.workflow(data) came from.

We currently have defaults, and they are:

• 2/3 of data allotted to training preproc + model
• randomly splitting data