chore: better bind_property implementation

[Rich-Harris]

Noticed our bind_property implementation has plenty of room for improvement:

• handler body is repeated
• we create a render effect for all bindings, when we only need one for those on window/document/document.body
• we have a "get"/"set" type which seems like a pretty strange way to solve the problem of the distinction between readonly and two-way bindings
• we always pass in a get_value even for readonly bindings

Given this (contrived) code:

<script>
  let w = $state(0);
</script>

<img bind:naturalWidth={w} />

Before:

$.bind_property("naturalWidth", "load", "get", img, () => $.get(w), ($$value) => $.set(w, $$value));

After:

$.bind_property("naturalWidth", "load", img, ($$value) => $.set(w, $$value));

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