fix(checkbox): bind indeterminate for Svelte 5 compatibility

[metonym]

Fixes #2039

In Svelte 5, <input indeterminate={indeterminate} /> does not update the value. Binding the variable seems to work.

Tested the following with Svelte 4 and Svelte 5. The difference now is that indeterminate is reactive (two-way).

<script>
  import { Checkbox } from "carbon-components-svelte";

  let indeterminate = true;
</script>

<Checkbox labelText="Label text" bind:indeterminate />

<p>Indeterminate: {indeterminate}</p>

<button on:click={() => (indeterminate = !indeterminate)}>Toggle indeterminate</button>

---

indeterminate.mov

[metonym]

Released in v0.86.0