Svelte 5: `$host()` rune for custom elements

[Rich-Harris]

Describe the problem

Previously: #3091

The current workaround for getting hold of the host element, when compiling as a custom element, is convoluted and weird.

Describe the proposed solution

It would be useful to have a $host() rune, which would be a compile error without the relevant option.

With this, we could treat createEventDispatcher as deprecated — IIRC the only real reason it isn't already is that we need it to dispatch events from custom elements.

Importance

nice to have