Export "Metadata" type so that consumers can extend it

Right now there is no way for consumers of this library to extend the metadata property in SharedFormProperties because SharedFormProperties isn't exported from the library.

Current Type Definition

// ./src/scheam.ts line 112
interface SharedFormProperties {
  definitions?: { [definition: string]: Schema };
  metadata?: { [name: string]: unknown };
  nullable?: boolean;
}

Proposed Change

export interface SharedFormProperties {
  definitions?: { [definition: string]: Schema };
  metadata?: SchemaMetadata;
  nullable?: boolean;
}

export interface SchemaMetadata {
  [key: string]: unknown;
}

Reasoning

In doing this tools and apps that consume this library can define type hints for metadata that they use similar to how people extend the request object in Fastify or Express. (See an example here: https://github.com/fastify/help/issues/122#issuecomment-915263438)

Here is an example of what this would allow. Writing a .d.ts file like the following.

// jtd.d.ts
import "jtd"
declare module "jtd" {
  export interface SchemaMetadata {
     id?: string;
     description?: string;
  }
}

Results in type hints like this:

jsontypedef / json-typedef-js