fix: register iconset when vaadin-iconset is defined

[javier-godoy]

I'm not sure if this is a fix to #105, but certainly it doesn't harm.

+customElements.whenDefined('vaadin-iconset').then(Iconset=>{
Iconset.register('fas', 1000, template);

const iconset = Iconset.getIconset('fas');

for (const name in aliases) {
    iconset._icons[name] = iconset._icons[aliases[name]];
}
+});

[paodb]

LGTM