@unovis/svelte: Fixing SingleContainer props and adding support for class property

[reb-dev]

closes: #291

Previously, $$restProps object was ignored in the initChart call, resulting in missing props. I fixed the logic and did some minor refactoring to ensure update calls weren't being called unnecessarily.

Additionally, users can now add the class property for containers.

Note: Due to the way svelte component styles are scoped, the :global modifier is required for it to take effect. It is recommended to combine local and global to avoid conflicts and keep the scope as limited as possible. For example:

<div>
    <VisSingleContainer class='chart'>...</VisSingleContainer>
</div>
<style>
    div :global(.chart) {
        /* Applies to elements in any component inside a <div>
        element belonging to this component. ✅ Recommended */
    }

    :global(.chart) {
        /* Applies to all elements in any component in the document
        where class="chart". ⛔ Works, but not recommended */
     }

    .chart {
        /* Strictly applies to elements (not components) that belong to this
        component. ❌ Doesn't work for `@unovis/svelte` containers */
    }
</style>