Add support for event properties

[shoonia]

When I use event.target on handler action, the types don't find the properties

for example:

$w('#myCheckbox').onChange((event) => {
    if (event.target.checked) { // <<< here
        // do ...
    } else {
        // do ...
    }
});

I see an ts error

Property 'checked' does not exist on type 'Element'.ts(2339)

Thanks!