Fix issue with typings when creating wrapper function

[Stinobe]

I noticed that when creating a wrapper function the types were acting up and only ZodType was working well

[Copilot]

Pull request overview

This PR adjusts the library’s public TypeScript typings to behave more predictably when consumers create wrapper functions around mattr/parse, and aligns excerpt function typing with actual runtime behavior.

Changes:

• Allow custom excerpt functions (ExcerptFunction) to return null.
• Simplify parse and extractExcerpt typings by removing ZodType-specific overload signatures.
• Add a patch changeset for the release.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/types.ts	Updates ExcerptFunction return type to string | null to match runtime behavior.
src/parse.ts	Removes overload signatures and consolidates parse into a single generic signature.
src/excerpt.ts	Removes ZodType overload signatures for extractExcerpt and unused import.
.changeset/soft-experts-boil.md	Adds a patch release note for the typing fix.

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