@typescript-eslint/no-unused-vars does not behave well with interfaces

[bcoe]

I noticed when switching yargs to use gts for linting, that variables inside an interface are flagged as unused by no-unused-vars:

export interface YargsInstance {
  $0: string;
  argv: Arguments;
  customScriptName: boolean;
  parsed: DetailedArguments | false;
  // The methods below are called after the parse in yargs-parser is complete
  // and perform cleanup on the object generated:
  _postProcess<T extends Arguments | Promise<Arguments>>(
    argv: T,
    populateDoubleDash: boolean,
    calledFromCommand?: boolean
  ): T;
  _copyDoubleDash<T extends Arguments>(argv: T): T;
  _parsePositionalNumbers<T extends Arguments>(argv: T): T;

Will lead to warnings like:

  1748:5   warning  'populateDoubleDash' is defined but never used     @typescript-eslint/no-unused-vars

The actual definition of self._postProcess does have the populateDoubleDash variable, and uses it.

Refs: yargs/yargs#1777