String formats validation and custom formats?

Hello, I'm using Cabidela in a Cloudflare Pages worker project but I don't think it handles any string-format validation yet does it?

I see that [pattern validation](https://github.com/cloudflare/cabidela/blob/d17218f718dd9836b491e0e74ecf50cab7edff64/src/index.ts#L377) is implemented, but I want to register a named string format and supply a validator function for it similar to how you do it for [TypeBox and Ajv](https://github.com/sinclairzx81/typebox?tab=readme-ov-file#formatregistry). Just curious if you could tell me how you might expose that feature?

It looks like the correct place to add calls to a validator would be around [here](https://github.com/cloudflare/cabidela/blob/d17218f718dd9836b491e0e74ecf50cab7edff64/src/index.ts#L346). Beyond that, I guess you would add a mapping of string-format validators in the `CabidelaOptions`, something like `formats: Record boolean | string>;`?

My main question about any possible future format validators is the function signature. I suppose I would be safe just returning a `boolean` to `Cabidela`, but that would limit the resulting error messages to `'${pathToString(needle.path)}' is not a valid '${format}'.` and it would be nice, if the validator returned a string, to do something like `'${pathToString(needle.path)}' is not a valid '${format}'. (${result})` where `result` would be the string returned from the validator.

For now I'll start by making my own validator functions just return booleans to be safe.

EDIT: The benefit of just returning a bool will be that I can use some code from [ajv-formats](https://www.npmjs.com/package/ajv-formats), so I guess that should be the standard anyway.

This addition was pretty straightforward, but if you're taking pull requests let me know and I'll send one.