pos-router manually set default uri

[jg10-mastodon-social]

pos-router currently sets the default uri in two modes

PodOS/elements/src/components/pos-router/pos-router.tsx

Lines 17 to 22 in ad3077a

	/**
	* The mode defines what default URI will be used, if no URI param is given
	*
	* - standalone: reroute to pod-os:dashboard
	* - pod: reroute to the URI that is shown in the actual browser
	*/

As a creator of an app, I want to be able to have the app start at a resource I specify.

This could be done with a new attribute, e.g. <pos-router default-uri="https://my.app/starting/resource">

A work around is to dispatch a pod-os:link event on pos-router on appload, to override the default uri as soon as it is loaded.