Update web-dialog.ts

[hansallis]

Hi,

First of all thanks for creating this module. I ran into an issue recently with Chrome. Instantiating a web-dialog element programmatically caused the following error: Uncaught DOMException: Failed to construct 'CustomElement': The result must not have attributes at <anonymous>:1:10

To mitigate this I've moved setting the aria-modal attribute from the constructor to the connectedCallback method.

Hope this constitutes an improvement.

Regards, Hans