Type definition of `JsonSchemaToOpenapiSchema` should return a Promise

[robertbullen]

Shouldn't the return type be a Promise? i.e.

  declare function JsonSchemaToOpenapiSchema<T = Record<string | number, any>>(
    schema: Record<string | number, any>,
    options?: JsonSchemaToOpenapiSchema.Options
- ): T;
+ ): Promise<T>;

Originally posted by @apancutt in #21 (comment)

[robertbullen]

Looks like #23 addresses this.