Formalizing a `<action_fns>` object

[yjunechoe]

As part of the action levels overhaul (#611), I wonder whether we could introduce a lightweight action_fns() object, such that:

action_levels(
  warn = 1,
  fns = list(warn = ~stock_warning(x = x))
)

Becomes:

action_levels(
  warn = 1,
  fns = action_fns(warn = ~stock_warning(x = x))
)

The most obvious benefit is it'd streamline the deprecation process both for the internal code (fns can just check whether it's receiving a <action_fns> object) and for hand-holding users (we can just direct them to use fns = action_fns() instead, which will host the relevant documentation).

Beyond that, I think having a function is nice because it obviates the need to checks for "list must be named", "list names must be one of xyz", etc. :)

[rich-iannone]

This is great! Having to force the user to carefully create a list with the correct structure and names is not good at all. This helper function will help a lot. Are you thinking of slow deprecating the previous method of input? I think that would be good to do.

[yjunechoe]

Are you thinking of slow deprecating the previous method of input?

I think so, yeah! So moving forward, if fns is not a <action_fns> object we can signal a deprecation warning and point users to use action_fns() before proceeding as normal, ensuring backwards compatibility