Register actions statically / globally (#3264)

This updates our approach to action registration to make it
static/global.

There are 3 different approaches to creating an action, depending on the
complexity of your action's implementation. All of them involve defining
a data type with the correct trait implementations and registering it,
each a bit more powerful / verbose.

* Define a simple list of unit structs that implement `Action` -
`actions!(Foo, Bar, Baz)`
* Make a more complex data type into an action with `#[action]`. This
derives all the necessary traits and registers the action.
  ```rs
  #[action]
  struct MoveLeft {
    word: true
  }
  ```
* Implement all traits yourself and just register the action with
`#[register_action]`.

Release Notes:

N/A