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, do something similar to this to notify Shiny of a change:

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

These boolean values map to values of priority: "deferred" (true) and priority: "immediate" (false), meaning 'send later & debounce' or 'send now'.

With this PR, you can now:

1. Specify the priority as an object, making the code more readable/explicit (i.e., callback({priority: "deferred"})
2. Provide a new value of {priority: "event"} to always send now, even if the value hasn't changed.

For example:

subscribe = function(el, callback) {
  $(el).on("keyup.exampleComponentName", function(event) {
    callback({priority: "event"})
  });
}

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]

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"})

[gadenbuie]

Looks great! Just a small note to make sure the PR includes a demo snippet with the new feature