Add ability to reflect prop changes back to attributes

Describe the problem

Currently it's not possible to reflect prop changes in the custom element attributes.

Describe the proposed solution

Due limitations caused by the fact that svelte-retag isn't able to hook into Svelte compiler options (at least not for now), the developer must first manually enable accessors in their component like so:

<svelte:options accessors />

svelte-retag would then add a new props option (if not already present, see #13) and include a reflect option, e.g.

svelteRetag({
  component: HelloWorld,
  tagname: 'hello-world',
  props: {
    greetPerson: { reflect: true },
  }
});

Once configured, modifications to greetPerson from within the component would be reflected in the greetperson custom element attribute.

Implementation Details

The attribute MutationObserver should be aware of reflected changes to prevent infinite loops.
See if possible to automate setting of accessors option if already hooking into compiler process

To watch for updates, one would have to utilize the after_updates array available on the component instance, e.g.

tag.componentInstance.$$.after_update.push(() => {
tag.setAttribute('greetperson', tag.componentInstance.greetPerson);
});

Alternatives considered

None.

Importance

nice to have

patricknelson / svelte-retag