Don't export a base class

[stevenvachon]

The problem with this project's exported WebComponent is that it extends HTMLElement when a use case may need to extend something else, such as HTMLInputElement.

Instead, this project should export simple functions:

import {getNumberAttribute} from 'webcomponent';

class MyComponent extends HTMLInputElement {
    something() {
        const value = getNumberAttribute(this, "attr-name");
    }
}