move methods for tunable() from tune to parsnip

[hfrick]

This PR is part of moving methods for tunable() from tune to the package which holds the corresponding object class:

• the methods for recipe, step, and check are already in recipes, that PR just cleans up the documentation. Clean up (internal) documentation of tunable() methods recipes#856
• the methods for model_spec and various model types such as linear_reg etc are moved to parsnip (aka this PR here)
• the method for workflow is moved to workflows move method for tunable() from tune to workflows workflows#130

When methods have been moved, the methods are registered conditionally on the version of tune from which they have been removed. The tune PR is tidymodels/tune#433

The tests require multiple packages so they are all in extratests: tidymodels/extratests#34

The PR on extratests should be merged first, then the ones on recipes and parsnip, then workflows, then tune.

This is part of the wider aim of introducing extract_parameter_dials() and extract_parameter_set_dials() and the next steps are:

• introducing extract_parameter_set_dials()
• introducing extract_parameter_dials()
• moving the methods for parameters() similar to this PR set
• updating documentation and learning materials

[github-actions]

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