feat: Allow schema metadata to be strongly typed

[shaungrady]

Resolves #2018.

Allows schema metadata to be strongly typed through declaration merging. From the updated README.md:

A custom SchemaMetadata interface can be defined through
merging
with the CustomSchemaMetadata interface. Start by creating a yup.d.ts file
in your package and creating your desired CustomSchemaMetadata interface:

// yup.d.ts
import 'yup';
declare module 'yup' {
  // Define your desired `SchemaMetadata` interface by merging the
  // `CustomSchemaMetadata` interface.
  export interface CustomSchemaMetadata {
    placeholderText?: string
    tooltipText?: string
    // …
  }
}

[jquense]

thanks!