feat: add support for serializable option in shadow DOM

[fusionstrings]

Adds support for the serializable option when creating shadow DOM. This helps serialising shadow DOM for server-side rendering scenarios.
This change is backward compatible

Changes

• Added serializable?: boolean option
• Added test coverage for the new option

Usage

register(MyComponent, 'my-element', [], { 
    shadow: true, 
    serializable: true 
});

Reference

• https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot/serializable
• https://jakelazaroff.com/words/isomorphic-web-components/

[rschristian]

Cheers