revisiting `tunable()` for model specifications

[simonpcouch]

Going ahead and PRing now to introduce tests and run GHA with them so I can demonstrate they still pass once I make some upcoming changes.

EDIT: Closes #1104.

[simonpcouch]

extract_parameter_set_dials() tests fail because they expect anything marked with tune() to appear in tunable() output.

Next steps would be to:

• Fill out the component_id() column in tune_args() instead of filling with NAs, and
• Redefine the join as a left join in extract_parameter_set_dials.model_spec() and look for the component_id in tune_args().

If those changes restore previous extract_parameter_set_dials.model_spec() behavior, the next step would be to revise and test tunable() methods in parsnip extensions. :)