Also mark `priority` as optional since it was optionally provided in practice

These classes create the custom Shiny events and help us extend JQuery's event handlers for these event types.

We now do this in place rather than as a separate function,
i.e. we replace `triggerFileInputChanged()` with our improved
abstraction.

…her than `evt`

It turns out the props on EventCommon aren't used everywhere

It was already used by preset-typescript, but we need to use the
plugin so that the transpilation happens at the right time, in
particular because we are using `declare` within classes.

* Use getters to expose data from `this.event` as if they were top-level
* Refer to the interfaces directly rather than repeating type definitions