feat: Better graphql-js interop, add mergeSchema to consume external schemas

[tgriesser]

Fixes: #673, #88, #106, #70

• Adds a new feature mergeSchema to the makeSchema config, to consume an external schema and merge with the locally defined Nexus types
• Standardizes the consumption of external GraphQLNamedType's so they are converted into Nexus type definitions, simplifying code paths
• Order of resolution goes from local Nexus types -> graphql-js types -> external schema types
• Adds asNexusMethod to all types, allowing for any commonly used types to be used as methods, not just scalars
• Convert methods / properties from protected -> private in Builder to better detect unused code

config.mergeSchema

export interface MergeSchemaConfig {
  /**
   * GraphQL Schema to merge into the Nexus type definitions.
   *
   * We unwrap each type, preserve the "nullable/nonNull" status of any fields &
   * arguments, and then combine with the local Nexus GraphQL types.
   *
   * If you have multiple schemas
   */
  schema: GraphQLSchema
  /**
   * If we want to "merge" specific types, provide a list of the types you wish to merge here.
   *
   * @default ['Query', 'Mutation']
   */
  mergeTypes?: string[] | true
  /**
   * If there are types that we don't want to include from the external schema in our final
   * Nexus generated schema, provide them here.
   */
  skipTypes?: string[]
  /**
   * If there are certain "fields" that we want to skip, we can specify
   * the fields here and we'll ensure they don't get merged into the schema
   */
  skipFields?: Record<string, string[]>
  /**
   * If there are certain arguments for any type fields that we want to skip, we can specify
   * the fields here & ensure they don't get merged into the final schema.
   *
   * @example
   *   skipArgs: {
   *     Mutation: {
   *       createAccount: ['internalId']
   *     }
   *   }
   */
  skipArgs?: Record<string, Record<string, string[]>>
}