feat(InputBinding): subscribe callback now supports event priority

[cpsievert]

Until now, custom input bindings could only pass a boolean value to their subscribe callback. That is, they all do something similar to this to notify Shiny of a change:

exampleInputBinding.subscribe = function(el, callback) {
  $(el).on("keyup.exampleComponentName", function(event) {
    callback(true); // or false
  });
}

These boolean values effectively map to values of priority: "deferred" (true) and priority: "immediate" (false).

This means, if an input wants to resend their value immediately regardless of whether the value has changed (i.e., priority: "event"), that isn't possible (until now).

The motivator for adding this is for input_submit_textarea() in rstudio/bslib#1204, where it feels like the most desirable behavior is to always resend immediately.

[gadenbuie]

If I'm reading the source correctly, InputPolicyOpts["priority"] is the same as EventPriority, right? If so, I think the second option is clearer because it doesn't require redirecting through InputPolicyOpts

[gadenbuie]

These boolean values effectively map to values of priority: "deferred" (true) and priority: "immediate" (false).

This explanation is the first time I've seen callback(false) or callback(true) written out in a way that is clear, concise and makes sense to me. JavaScript's positional variables are inherently opaque, but the naming here doesn't help.

If we're updating the API, I'd love to push it in a direction that supports a more readable interface. I.e. in addition to backcompat for callback(<boolean>), it'd be nice if the augmented API could support

callback(true)
callback({ priority: "deferred" })

callback(false)
callback({ priority: "immediate" })

// or now also
callback({priority: "event"})