feat: accept Web API Request object in enableAutoPreviewsFromReq()

[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 Web API Request objects containing a Web API Headers instance. Frameworks like SvelteKit use this API rather than an Express-style request object which, unlike the Web API counterpart, contains plain-object-based data.

import * as prismic from "@prismicio/client";

const headers = new Headers();
headers.set("cookie", `io.prismic.preview=abc123`);

const request = {
  headers,
  url: 'https://example.com/preview?documentId=abc123&token=abc123'
};

const client = prismic.createClient("qwerty");
client.enableAutoPreviewsFromReq(request);

// The client will automatically read the `io.prismic.preview` cookie
// and URL parameters to enable preview sessions.

For more details, see #239.

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

Merging #240 (6a0fa54) into master (f93af08) will decrease coverage by 0.26%.
The diff coverage is 95.65%.

@@             Coverage Diff             @@
##            master     #240      +/-   ##
===========================================
- Coverage   100.00%   99.73%   -0.27%     
===========================================
  Files           23       23              
  Lines          363      372       +9     
  Branches        67       74       +7     
===========================================
+ Hits           363      371       +8     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
src/client.ts	99.54% <95.45%> (-0.46%)	⬇️
src/lib/getCookie.ts	100.00% <100.00%> (ø)

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[github-actions]

size-limit report 📦

Path	Size
dist/index.js	4.06 KB (+1.12% 🔺)
dist/index.cjs	6.93 KB (+0.63% 🔺)

[lihbr]

LGTM, just a small one to make it stronger ☺️