New rule: unused generic type parameter

[lostfictions]

For example, in the Redux type definitions we see this:

export interface Dispatch<S> {
    <A extends Action>(action: A): A;
}

S is unused and only adds confusion and friction for consumers of this interface! It'd be useful to be able to show a warning in cases like this.

[myitcv]

👍

For some reason I thought there was already an open issue for this, but I can't find one so thanks for opening @lostfictions

For useful background on the issues this creates see microsoft/TypeScript#7797 amongst other issues

[andy-hanson]

Should be fixed by #2235, as the TypeScript implementation of no-unused-locals is used now.