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.