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

[angeloashmore]

Types of changes

• Chore (a non-breaking change which is related to package maintenance)
• 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:

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

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

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

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

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Additional details and impacted files

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

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