fix(webcomponents): make types play nicely with lit-element

Before this change, returning html`<sometag>...</sometag>` from `storiesOf(...).add` would result in the following error: Type 'TemplateResult' is not assignable to type 'StoryFnHtmlReturnType'

app/web-components/src/client/preview/types.ts

@@ -1,6 +1,7 @@
 import { StoryFn } from '@storybook/addons';
+import { TemplateResult, SVGTemplateResult } from 'lit-element';
 
-export type StoryFnHtmlReturnType = string | Node;
+export type StoryFnHtmlReturnType = string | Node | TemplateResult | SVGTemplateResult;
 
 export interface IStorybookStory {
   name: string;