feat: Add empty types/index.d.ts file

[TSenter]

It is often necessary for TypeScript projects to include ambient types, or to at least declare/override existing types (for example, an Embroider config). A few examples:

1. Providing types for packages/addons not written in TypeScript
2. Providing types for your own Embroider config options

declare module '@embroider/macros' {
  interface MyConfig {
    foo: string;
    bar?: boolean;
  }

  export function getOwnConfig(): MyConfig | undefined;
}

I consider this file akin to the unpublished-development-types/index.d.ts file provided by @ember/addon-blueprint. For example, in my company's @nrg-ui/core addon, we use these types for our own Embroider config, like so:

export interface EmbroiderConfig {
  appVersion: string;
  breakpoints: {
    xsmall: string;
    small: string;
    medium: string;
    large: string;
    xlarge: string;
    xxlarge: string;
  };
}

declare module '@embroider/macros' {
  export function getOwnConfig(): Partial<EmbroiderConfig> | undefined;
}