[typescript-fetch] Use ES2015 for typescript compiling

Description

Better to use ES2015 typescript compiling module for webpack/rollup.js import optimization.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/typescript-fetch/package.mustache

Command line used for generation

N/A

Steps to reproduce

N/A

Related issues/PRs

N/A

Suggest a fix/enhancement

My suggestion is adding module and jsnext:main in package.json point to the new file used es2015. ( like this: https://github.com/rollup/rollup/wiki/pkg.module )

Remaining the main points to the file used commonjs, we can use it same as before.

{
  ...
  "main": "./dist/index.js",
  "module": "./es/index.js",
  "jsnext:main": "./es/index.js",
  "typings": "./dist/index.d.ts",
  "scripts" : {
    "build": "tsc --outDir dist/",
    "build:es2015": "tsc --outDir es/ --module ES2015",
    "prepublishOnly": "npm run build && npm run build:es2015"
  }
  ...
}

Start with the typescript-fetch package.mustache template linked in the issue and review how its package metadata and build scripts are generated. Verify that CommonJS remains the main output while an ES2015 build is exposed through module and jsnext:main; done means the generated package includes both outputs and the corresponding scripts.