BREAKING: Icon definition requires a function using a tag passed via arguments:
```js
// myicon.svg.js
// before
export default '<svg>...</svg>';
// after
export default tag => tag`<svg>...</svg>`;
```
Application developers have an alternative shortcut to use in-place with lit-html:
```js
// MyComponent.js
// before
render() {
return html`
<lion-icon .svg="${'<svg>...</svg>'}"></lion-icon>
`;
}
// after
render() {
return html`
<lion-icon .svg="${html`<svg>...</svg>`}"></lion-icon>
`;
}
```
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