feat(SliceZone): add support for mapped Slice Zones created with `@prismicio/client`'s `unstable_mapSliceZone()`

Types of changes

[ ] Chore (a non-breaking change which is related to package maintenance)
[ ] Bug fix (a non-breaking change which fixes an issue)
[x] New feature (a non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR adds support for @prismicio/client's mapSliceZone() to <SliceZone>.

mapSliceZone() adds a __mapped property used to detect mapped Slices. <SliceZone> detects the __mapped property and automatically spreads the Slice's properties as props. An unmapped Slice will continue to provide the standard { slice, index, slices, context } props.

Example

// +page.server.js

import { mapSliceZone } from "@prismicio/client";
import { createClient } from "$lib/prismicio";

export async function load({ params, fetch, cookies }) {
  const client = createClient({ fetch, cookies });
  const page = await client.getByUID("page", params.uid);

  const slices = await mapSliceZone(
    page.data.slices,
    {
      foo: () => ({ foo: "bar" }),
      bar: () => ({ baz: "qux" }),
    },
  );

  return { slices };
}

<!-- +page.svelte -->

<script>
    import { SliceZone } from '@prismicio/svelte';

    import { components } from '$lib/slices';

    export let data;
</script>

<!-- The `foo` and `bar` components will receive the props returned in the functions above. -->
<SliceZone slices={data.slices} {components} />

Checklist:

[x] My change requires an update to the official documentation.
[x] All TSDoc comments are up-to-date and new ones have been added where necessary.
[x] All new and existing tests are passing.

🦆

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.62%. Comparing base (8c00747) to head (8c9bbb9).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #19 +/- ## ========================================== + Coverage 67.11% 67.62% +0.50% ========================================== Files 18 18 Lines 894 908 +14 Branches 1 1 ========================================== + Hits 600 614 +14 Misses 294 294 ```

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prismicio / prismic-svelte