Adding 'independent' mode for getStaticPaths

[wjaykim]

Goals

1. Make dynamic path page to be rendered(SSG) in build time only intentionally

Non-Goals

Background

Already discussed in #17772, there is a need for getStaticProps without getStaticPaths when we define getStaticProps which does not depend on the dynamic path parameter.

Since the page does not depend on parameter but renders same output for different dynamic path, developers may don't want getStaticPaths to run on runtime.

So I want nextjs to make dynamic path page to be rendered(SSG) in build time only intentionally if some option is specified.

Proposal

Defining getStaticPaths with some new option like:

export const getStaticPaths = (() => 'independent') satisfies GetStaticPaths;

or

export const getStaticPaths = (() => ({
  paths: 'independent',
})) satisfies GetStaticPaths;

When one of those new option is specified, make getStaticProps to not run on runtime, but only in buildtime.