feat: implement support for vanilla JS components in `onRenderValue` (WIP)

[josdejong]

Implements a solution for #375, this allows to use vanilla JS components in onRenderValue by defining a Svelte Action:

export interface MyComponentProps {
  // ...
}

export const MyComponent = Action<HTMLDivElement, MyComponentProps> = (node, props) => {
  // ...
  return {
    update: (props) => {
      // ...
    },
    destroy: () => {
      // ...
    }
  }
}

TODO:

• [x] Fix TypeScript issues
• [x] Write docs

[josdejong]

Published now in v0.21.5