fix(kit): support previews using a `/preview/` route prefix

[angeloashmore]

Types of changes

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

Description

This PR updates the existing @prismicio/svelte/kit preview functions and components to support a new, cleaner method of integrating Prismic previews.

• redirectToPreviewURL() now prefixes routes with /preview/ (configurable), which enables server rendering.
• <PrismicPreview> now adds a special navigation handler to prefix links with /preview/ (configurable), which enables server rendering.

Documentation on the recommended SvelteKit app setup and @prismicio/svelte/kit's API will be published soon.

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-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b88da24) 98.00% compared to head (776b359) 98.00%.

:exclamation: Current head 776b359 differs from pull request most recent head 2508778. Consider uploading reports for the commit 2508778 to get more accurate results

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #16 +/- ## ======================================= Coverage 98.00% 98.00% ======================================= Files 11 11 Lines 600 600 ======================================= Hits 588 588 Misses 12 12 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.