feat: allow reusing resolvers

[GiteshKhanna]

This PR exports resolver functions so we can reuse them. Let me give you an example:-

I wanted to use arraysUnion on enum keyword. But the only option I had was to either

1. Copy paste the arraysUnion function & pass it in mergeSchemas function. like

const mergedSchema = schemaMerger.mergeSchemas([schema1,schema2], {
resolvers: {
enum: CustomResolvers.arraysUnion
}
});

(here CustomResolvers is the module I created in my repo where I've copy pasted your arraysUnion function)

2. Hacky reference import, because I know that required is using arraysUnion as a resolver.

const mergedSchema = schemaMerger.mergeSchemas([schema1, schema2], {
resolvers: {
enum: schemaMerger.keywordsResolvers.required
}
} );

But now, after this change, on exporting resolvers, it makes your resolver functions re-usable. Hence I could do something like below, which is very intuitive and reduces boilerplate code for someone using the lib:-

const mergedSchema = schemaMerger.mergeSchemas([schema1, schema2], {
resolvers: {
enum: schemaMerger.resolvers.arraysUnion
}
} );

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Please add a test for this export as well as the types.
The types tests live in:
https://github.com/fastify/merge-json-schemas/blob/main/types/index.test-d.ts

[mcollina]

Docs are also needed

[mcollina]

lgtm

[Copilot]

Pull Request Overview

This PR exports the resolver functions to allow users to reuse them easily instead of resorting to copy-pasting or hacky imports.

• Exports the resolvers from index.js
• Updates type tests to verify resolver types
• Adds unit tests and README documentation to demonstrate usage

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

File	Description
types/index.test-d.ts	Updated import and type tests to ensure exported resolvers work
test/exports.test.js	New tests verifying the resolvers export and type function
index.js	Modified export to include resolvers
README.md	Documentation updated with an example of resolver usage

[GiteshKhanna]

Thanks for the reviews @Fdawgs @mcollina! Can this be merged & a version be released for the same?

[Fdawgs]

@ivan-tymoshenko wdyt?

[GiteshKhanna]

Hi @ivan-tymoshenko — just a gentle nudge on this one. The merge is currently blocked and I’ve already got two approvals. Could you please take a look when you get a moment? Appreciate your help!