feat(ui5-icon): use SVG icons instead of icon font

[pskelin]

• ui5-icon now uses SVGs (Scalable Vector Graphics) as they allow vector graphics to be displayed in the browser. SVGs are quickly becoming the new standard for web icons and animations. They not only offer superior scaling, but they often render more quickly and reliably than icon fonts.
• And, you can now import only the icons you actually need, which will reduce the overall bundle size.

BREAKING CHANGE:
To display an icon, you should explicitly import the desired one(s), as shown below:

import "@ui5/webcomponents-base/dist/Icon";
import "@ui5/webcomponents-base/dist/icons/slim-arrow-left.js";

<ui5-icon src="sap-icon://slim-arrow-left"></ui5-icon>

If you indirectly use icons via other components API, you should import the icons as well:

import "@ui5/webcomponents-base/dist/icons/slim-arrow-left.js";

<ui5-button icon="sap-icon://slim-arrow-left"></ui5-button>
<ui5-li icon="sap-icon://slim-arrow-left"></ui5-li>