Add pathPrefix support for local storage mode

[m4rrc0]

This PR aims to solve Thinkmill/keystatic#1431.

It allows using a pathPrefix in local mode, which is useful for editing content files outside of the project root for example but is not limited to this use case.

It is designed to have minimal footprint on the rest of the codebase.

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Latest commit: 55b144d828bff3866f7e1d321db41cdaa0e8c297

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[emmatown]

Providing a localBaseDirectory to the API route is the intended way to do this

[m4rrc0]

@emmatown , could you elaborate with an example or point me to some resource please, since it is not documented? How should I configure this?

I noticed the localBaseDirectory but it seemed on a different level of abstraction... not really something meant to be easily customized in the config.