feat(typescript): allow unused vars starting with underscores

[francoischalifour]

Description

Keeping an unused variable in a function signature is sometimes useful to know what data we're dealing with. A common pattern is to prefix this variable with _ to make it explicit that it's not used but it's usable. This pattern is used in the InstantSearch codebases.

This rule is already used in the InstantSearch.js ESLint configuration.

Examples

function example(start: number): number[] {
  const values = ['a', 'b', 'c'];

  return values.map((_, index) => start + index);
}