Add lint check for unused imports

[cramforce]

Imports should be used or not done.

CC @erwinmombay

[erwinmombay]

So we can't really check for unused imports since imports can have side effects but per the conversation in https://github.com/ampproject/amphtml/pull/1211/files, we can turn on unused reference lint checks but it currently errors out cause it also checks for unused properties and we have the following scattered all over our code base.

class SomeClass {
  constructor() {
    /** @private */
    this.someProp_;
  }
}

I'll have to do a cleanup of that first, but i 100% agree this should be on.

[cramforce]

Yeah, we should really change those by adding = undefined.

I think that check would really give us robustness!

[erwinmombay]

i stand corrected, the one i stated was for unused expression. but where it actually errors out are on our typedefs, like so:

/**
 * @typedef {{
 *   newStackIndex: number
 * }}
 */
let ViewerHistoryPoppedEvent;

now we need to add either a Def suffix or a Interface suffix so that they are ignored by the linter. (for typdefs, templates and class interfaces)