fix: Allow to use bind:property on custom elements

[ouvreboite]

🐛 Related issue #4838

Hello Svelte team,

Currently a compilation error is thrown when trying to bind on a (valid or invalid) property from a custom element (aka web component).
For example:

<my-custom-element bind:value={myValue}></my-custom-element>
<!-- The compiler returns a validation error: 'value' is not a valid binding on <my-custom-element> -->

The goal of this PR is to propose a naive (and simple) fix for Svelte 4. A more robust solution can be implemented in Svelte 5, but some design discussion may be necessary.

The fix returns a warning (instead of an error) when using bind:property on custom elements. bind:this is still supported (no error or warning).

❓ 🆘 How to test properly?
I was able to adapt the compilation check easily and create a test case for the new warning being returned. But I don't know how to implement a proper test (using an actual web component, binding to one of its property and checking that the binding works). I welcome some feedback on that.

🧪 New test case for compilation validator

<script>
	let myValue;
	let myElement;
</script>

<my-element bind:value={myValue} bind:this={myElement}></my-element>

<!-- will return no error -->
<!-- will return a single warning: Binding on custom element <my-element> is not checked. You need to ensure 'value' is a valid property. -->

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[dummdidumm]

Thanks, but I'm not sure this is a good solution. It assumes that the bindings on custom elements behave exactly like those on the native dom elements they were originally designed for, which may not be the case. People will likely get confused by it not working, and custom element authors would now be on the hook for somehow adjusting their custom elements to account for someone using a binding. We framework authors would now also have to be extra careful about not assuming a native dom element or not using private properties as much.