feat: SvelteKit previews

[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 adds support for Prismic previews in SvelteKit. It adds a @prismicio/svelte/kit entry, which provides components and helpers to support previews.

• <PrismicPreview>: Renders the Prismic toolbar and adds SvelteKit-specific preview event handlers to update page data.
• enableAutoPreviews(): Configures a Prismic client to support SvelteKit-style previews.
• redirectToPreviewURL(): Creates a Response object to redirect a content writer to a previewed document.

Documentation on @prismicio/svelte/kit will be available 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 (65b0e7f) 92.67% compared to head (e67132c) 98.00%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   92.67%   98.00%   +5.32%     
==========================================
  Files          11       11              
  Lines         601      601              
==========================================
+ Hits          557      589      +32     
+ Misses         44       12      -32     

see 3 files with indirect coverage changes

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