Create "derivers," as an exception-free option for working with structured data in swatches.

This is meant to be used alongside `$derive`, for inputs with complex structure. For example:

```js
let jsonInput = $state('{}');
let json = $derived(deriver.json(jsonInput));

// …

<textarea bind:value={jsonInput}></textarea>
```

This allows textual editing of the data, while preventing exceptions due to syntax or logical errors in partially-edited data from breaking Svelte's derive process (see comments). Note that these exceptions are not considered [unexpected errors] by SvelteKit, because they do not arise "while handling a request;" they are considered errors by Svelte, but Svelte doesn't appear to provide any affordances for handling errors in this context, so we have to bring our own.

[unexpected errors]: https://svelte.dev/docs/kit/errors#Unexpected-errors

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