Avoid import when targets support native `fetch`

We can use this.project.targets to determine if the whatg-fetch polyfill is required for the browser builds, and avoid app.importing it when it is not needed.

See browser support. Roughly includes all evergreen browsers (FireFox, Edge, Chrome, Safari)…

[rwjblue]

@mydea - Yes, totally agreed.

@xg-wang - This issue was created to track avoiding the include the polyfill at all in vendor.js, but AFAICT the fix added in #63 will still continue to ship the polyfill but will avoid using it if native fetch is present. I’m reopening this issue so that we can track the vendor.js “slim down”…

[xg-wang]

Released as v6.4.0 🎉

As mentioned in readme, to avoid bundling redundant polyfill assets just set preferNative to true

// ember-cli-build.js
let app = new EmberAddon(defaults, {
  // Add options here
  'ember-fetch': {
    preferNative: true
  }
});

Browser targets supports for fetch and abortcontroller are checked from caniuse-api. ember-fetch will then load only the necessary polyfills and its own wrapper 😄

Thanks @mydea for landing this feature!