fix: export CredentialsSignin class to extend in custom authorize errors

[ndom91]

☕️ Reasoning

• Extending feat: customizable authorize() error #9871
• Explicitly shows throwing the custom error in the Credentials provider docs
• Exports the CredentialsSignin class you're meant to extend from each framework package, to avoid having to install @auth/core just for this.

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 4, 2024 4:43pm

2 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
next-auth-docs	⬜️ Ignored (Inspect)	Visit Preview		Mar 4, 2024 4:43pm
nextra-docs	⬜️ Ignored (Inspect)	Visit Preview		Mar 4, 2024 4:43pm

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 39.29%. Comparing base (0168f3c) to head (9a78395).

Files	Patch %	Lines
packages/frameworks-solid-start/src/index.ts	0.00%	9 Missing ⚠️
packages/frameworks-sveltekit/src/lib/index.ts	0.00%	2 Missing ⚠️
packages/next-auth/src/index.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10200      +/-   ##
==========================================
- Coverage   39.30%   39.29%   -0.01%     
==========================================
  Files         171      171              
  Lines       27242    27256      +14     
  Branches     1151     1151              
==========================================
+ Hits        10708    10711       +3     
- Misses      16534    16545      +11     

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