Tabs throw if a child tab is immediately appended

[damyanpetev]

Description

Tabs throw if a child tab is appended synchronously immediately after being added to DOM. This is caused by the constructor-initialized mutation observer/controller and it's handler referencing the internal resize observer which is created on firstUpdated, so if the mutation happens immediately after connecting to the DOM the resize observer doesn't exist yet.

Steps to reproduce

On https://igniteui.github.io/igniteui-webcomponents/?path=/story/tabs--basic or any page with registered tabs component
Run the following:

var tabs = document.createElement('igc-tabs');
var tab = document.createElement('igc-tab');
document.body.appendChild(tabs);
tabs.appendChild(tab);

Result

Expected result

There should be no error and the tabs should work as expected.