Update tsconfig to match the standard blueprint

[chriskrycho]

Either:

• copy directly from ember-cli-typescript's blueprint

or:

• use @tsconfig/ember if tsconfig/bases#102 lands soon enough

[SergeAstapov]

let's do this after #332 lands to reduce conflicts churn :)

[chriskrycho]

Yep. This came out of my leaving a comment there (but I'm still mid-review). 😂

[chriskrycho]

We should be able to yarn add -D @tsconfig/ember and do "extends": "@tsconfig/ember/tsconfig.json" in the config starting tomorrow. (The blueprint on the Ember side will be updated to do the same.)