Return the newly created HtmlElement class, as opposed to just "undef… (#91)

* Return the newly created HtmlElement class, as opposed to just "undefined"

Currently `register` returns the result of `customElements.define`, which happens to be undefined.

While it is possible to get the just defined tab right away, it would make sense to return the newly create HtmlElement class instead.

```js
const klass = register(Component, 'element-tag');
```

vs

```js
register(Component, 'element-tag');
const klass = customElements.get('element-tag');
```

* feat: Add types

---------

Co-authored-by: Ryan Christian <rchristian@ryanchristian.dev>

Author: EmmanuelOga

src/index.d.ts

@@ -43,11 +43,12 @@ type Options =
  *   shadow: true,
  *   mode: 'closed'
  * });
+ * const klass = register(PreactComponent, 'my-component');
  * ```
  */
 export default function register(
 	Component: AnyComponent,
 	tagName?: string,
 	propNames?: string[],
 	options?: Options
-): void;
+): HTMLElement;

src/index.js

@@ -75,10 +75,12 @@ export default function register(Component, tagName, propNames, options) {
 		});
 	});
 
-	return customElements.define(
+	customElements.define(
 		tagName || Component.tagName || Component.displayName || Component.name,
 		PreactElement
 	);
+
+	return PreactElement;
 }
 
 function ContextProvider(props) {