Add "Split all function arguments" code action

[isovector]

Another hot feature requested by @TOTBWF. This PR adds a new code action that performs a case split on all bound function arguments. For example, given:

and :: Bool -> Bool -> Bool
and = _

it will produce

and :: Bool -> Bool -> Bool
and False False = _
and True False = _
and False True = _
and True True = _

[googleson78]

A more modular approach taken in Agda is to be able specify which arguments to split on in the hole (or prompt the user to input them). So for Haskell it might look something like

f x y z = _x_y
-- case split action results in all the cases for x and y only

Not sure if we have hole info though, and if the "advanced hole info stuff" by @Tritlo even is merged in ghc.