Svelte 5: optimise prop access

[Rich-Harris]

Describe the problem

A component author might choose to use rest props as an authoring convenience:

<script>
-  let { a, b } = $props();
+  let { ...props } = $props();
</script>

-<p>{a} + {b} = {a + b}</p>
+<p>{props.a} + {props.b} = {props.a + props.b}</p>

It's not the pattern we promote (you can't specify fallback values or bindability), but it's a valid choice. Right now, this creates a rest_props proxy, even though it's not strictly necessary in this component:

let props = $.rest_props($$props, []);

Describe the proposed solution

If the only times props is referenced is in a non-computed member expression, we can skip the rest_props call and treat props.a as though there were a named a prop.

Importance

nice to have