Allow binding to custom elements

[redmagic]

Describe the problem

I am unable to bind to properties created by web components.

For example https://component.kitchen/elix/ListBox exposes a value attribute, but svelte does not allow <elix-list-box bind:value={someVariable}>

Describe the proposed solution

Maybe this check can be controlled by a compiler flag e.g. allowUnvalidatedBindings.

svelte/src/compiler/compile/nodes/Element.ts

Line 648 in 95f3f1c

if (

Alternatives considered

Get the desired value through a custom event listener, like so: <elix-list-box on:selectedindexchange={(e) => (someVariable = e.target.value)}>

This feel convoluted though and not confirming to svelte's design.

Importance

would make my life easier

[geoffrich]

Duplicate of #4838. There's discussion in that issue on how this should be implemented, since the current value binding works by listening to input and the correct event could vary depending on the web component implementation.