typescript codegen

[stephen]

This PR adds typescript codegen support to the loader. The option can be enabled with a codegen block:

options: {
  schema: "./schema.json",
  codegen: {
    typescript: {
      passthroughCustomScalars: true,
      customScalarsPrefix: "gql_",
    },
  },
},

The loader will then:
a) Emit a QuerySpec object for the query as the default export, along with the input/output types:

export interface FilmsQueryVariables {
  k: string,
};

export interface FilmsQuery {
  film:  {
    // The title of this film.
    title: string | null,
  } | null,
};

export interface Spec<Result extends object, Input extends object> {
  query: string;
  result?: Result;
  variables?: Input;
}

const documentOutput = "query FilmsQuery { ... }";
const Spec: Spec<FilmsQuery, FilmsQueryVariables> = { query: documentOutput };

The types are generated using apollo-codegen.

b) Write a .graphql.d.ts file to the filesystem as a sibling to the original .graphql file. This is useful for letting the IDE provide a useful type for the query.

Users of the feature may want to continue to put a:

declare module "*.graphql";

somewhere, so that things don't freak out before a type generation pass occurs.

Todo

• If variables don't exist, don't emit them.